diff --git a/python/pyspark/sql/functions.py b/python/pyspark/sql/functions.py
index 2694e777d8266..8b2e423d250cd 100644
--- a/python/pyspark/sql/functions.py
+++ b/python/pyspark/sql/functions.py
@@ -3023,6 +3023,42 @@ def pandas_udf(f=None, returnType=None, functionType=None):
         conversion on returned data. The conversion is not guaranteed to be correct and results
         should be checked for accuracy by users.
     """
+
+    # The following table shows most of Pandas data and SQL type conversions in Pandas UDFs that
+    # are not yet visible to the user. Some of behaviors are buggy and might be changed in the near
+    # future. The table might have to be eventually documented externally.
+    # Please see SPARK-25798's PR to see the codes in order to generate the table below.
+    #
+    # +-----------------------------+----------------------+----------+-------+--------+--------------------+--------------------+--------+---------+---------+---------+------------+------------+------------+-----------------------------------+-----------------------------------------------------+-----------------+--------------------+-----------------------------+-------------+-----------------+------------------+-----------+--------------------------------+  # noqa
+    # |SQL Type \ Pandas Value(Type)|None(object(NoneType))|True(bool)|1(int8)|1(int16)|            1(int32)|            1(int64)|1(uint8)|1(uint16)|1(uint32)|1(uint64)|1.0(float16)|1.0(float32)|1.0(float64)|1970-01-01 00:00:00(datetime64[ns])|1970-01-01 00:00:00-05:00(datetime64[ns, US/Eastern])|a(object(string))|  1(object(Decimal))|[1 2 3](object(array[int32]))|1.0(float128)|(1+0j)(complex64)|(1+0j)(complex128)|A(category)|1 days 00:00:00(timedelta64[ns])|  # noqa
+    # +-----------------------------+----------------------+----------+-------+--------+--------------------+--------------------+--------+---------+---------+---------+------------+------------+------------+-----------------------------------+-----------------------------------------------------+-----------------+--------------------+-----------------------------+-------------+-----------------+------------------+-----------+--------------------------------+  # noqa
+    # |                      boolean|                  None|      True|   True|    True|                True|                True|    True|     True|     True|     True|       False|       False|       False|                              False|                                                False|                X|                   X|                            X|        False|            False|             False|          X|                           False|  # noqa
+    # |                      tinyint|                  None|         1|      1|       1|                   1|                   1|       X|        X|        X|        X|           1|           1|           1|                                  X|                                                    X|                X|                   X|                            X|            X|                X|                 X|          0|                               X|  # noqa
+    # |                     smallint|                  None|         1|      1|       1|                   1|                   1|       1|        X|        X|        X|           1|           1|           1|                                  X|                                                    X|                X|                   X|                            X|            X|                X|                 X|          X|                               X|  # noqa
+    # |                          int|                  None|         1|      1|       1|                   1|                   1|       1|        1|        X|        X|           1|           1|           1|                                  X|                                                    X|                X|                   X|                            X|            X|                X|                 X|          X|                               X|  # noqa
+    # |                       bigint|                  None|         1|      1|       1|                   1|                   1|       1|        1|        1|        X|           1|           1|           1|                                  0|                                       18000000000000|                X|                   X|                            X|            X|                X|                 X|          X|                               X|  # noqa
+    # |                        float|                  None|       1.0|    1.0|     1.0|                 1.0|                 1.0|     1.0|      1.0|      1.0|      1.0|         1.0|         1.0|         1.0|                                  X|                                                    X|                X|1.401298464324817...|                            X|            X|                X|                 X|          X|                               X|  # noqa
+    # |                       double|                  None|       1.0|    1.0|     1.0|                 1.0|                 1.0|     1.0|      1.0|      1.0|      1.0|         1.0|         1.0|         1.0|                                  X|                                                    X|                X|                   X|                            X|            X|                X|                 X|          X|                               X|  # noqa
+    # |                         date|                  None|         X|      X|       X|datetime.date(197...|                   X|       X|        X|        X|        X|           X|           X|           X|               datetime.date(197...|                                                    X|                X|                   X|                            X|            X|                X|                 X|          X|                               X|  # noqa
+    # |                    timestamp|                  None|         X|      X|       X|                   X|datetime.datetime...|       X|        X|        X|        X|           X|           X|           X|               datetime.datetime...|                                 datetime.datetime...|                X|                   X|                            X|            X|                X|                 X|          X|                               X|  # noqa
+    # |                       string|                  None|       u''|u'\x01'| u'\x01'|             u'\x01'|             u'\x01'| u'\x01'|  u'\x01'|  u'\x01'|  u'\x01'|         u''|         u''|         u''|                                  X|                                                    X|             u'a'|                   X|                            X|          u''|              u''|               u''|          X|                               X|  # noqa
+    # |                decimal(10,0)|                  None|         X|      X|       X|                   X|                   X|       X|        X|        X|        X|           X|           X|           X|                                  X|                                                    X|                X|        Decimal('1')|                            X|            X|                X|                 X|          X|                               X|  # noqa
+    # |                   array<int>|                  None|         X|      X|       X|                   X|                   X|       X|        X|        X|        X|           X|           X|           X|                                  X|                                                    X|                X|                   X|                    [1, 2, 3]|            X|                X|                 X|          X|                               X|  # noqa
+    # |              map<string,int>|                     X|         X|      X|       X|                   X|                   X|       X|        X|        X|        X|           X|           X|           X|                                  X|                                                    X|                X|                   X|                            X|            X|                X|                 X|          X|                               X|  # noqa
+    # |               struct<_1:int>|                     X|         X|      X|       X|                   X|                   X|       X|        X|        X|        X|           X|           X|           X|                                  X|                                                    X|                X|                   X|                            X|            X|                X|                 X|          X|                               X|  # noqa
+    # |                       binary|                     X|         X|      X|       X|                   X|                   X|       X|        X|        X|        X|           X|           X|           X|                                  X|                                                    X|                X|                   X|                            X|            X|                X|                 X|          X|                               X|  # noqa
+    # +-----------------------------+----------------------+----------+-------+--------+--------------------+--------------------+--------+---------+---------+---------+------------+------------+------------+-----------------------------------+-----------------------------------------------------+-----------------+--------------------+-----------------------------+-------------+-----------------+------------------+-----------+--------------------------------+  # noqa
+    #
+    # Note: DDL formatted string is used for 'SQL Type' for simplicity. This string can be
+    #       used in `returnType`.
+    # Note: The values inside of the table are generated by `repr`.
+    # Note: Python 2 is used to generate this table since it is used to check the backward
+    #       compatibility often in practice.
+    # Note: Pandas 0.19.2 and PyArrow 0.9.0 are used.
+    # Note: Timezone is Singapore timezone.
+    # Note: 'X' means it throws an exception during the conversion.
+    # Note: 'binary' type is only supported with PyArrow 0.10.0+ (SPARK-23555).
+
     # decorator @pandas_udf(returnType, functionType)
     is_decorator = f is None or isinstance(f, (str, DataType))