diff --git a/ppl/src/main/java/org/opensearch/sql/ppl/parser/AstExpressionBuilder.java b/ppl/src/main/java/org/opensearch/sql/ppl/parser/AstExpressionBuilder.java
index 47db10c99b..f36765d3d7 100644
--- a/ppl/src/main/java/org/opensearch/sql/ppl/parser/AstExpressionBuilder.java
+++ b/ppl/src/main/java/org/opensearch/sql/ppl/parser/AstExpressionBuilder.java
@@ -187,7 +187,7 @@ public UnresolvedExpression visitTakeAggFunctionCall(
   /** Eval function. */
   @Override
   public UnresolvedExpression visitBooleanFunctionCall(BooleanFunctionCallContext ctx) {
-    final String functionName = ctx.conditionFunctionBase().getText();
+    final String functionName = ctx.conditionFunctionBase().getText().toLowerCase();
     return buildFunction(
         FUNCTION_NAME_MAPPING.getOrDefault(functionName, functionName),
         ctx.functionArgs().functionArg());
diff --git a/ppl/src/test/java/org/opensearch/sql/ppl/parser/AstExpressionBuilderTest.java b/ppl/src/test/java/org/opensearch/sql/ppl/parser/AstExpressionBuilderTest.java
index 7bcb87d193..de230a1fee 100644
--- a/ppl/src/test/java/org/opensearch/sql/ppl/parser/AstExpressionBuilderTest.java
+++ b/ppl/src/test/java/org/opensearch/sql/ppl/parser/AstExpressionBuilderTest.java
@@ -107,12 +107,15 @@ public void testLogicalLikeExpr() {
   @Test
   public void testBooleanIsNullFunction() {
     assertEqual("source=t isnull(a)", filter(relation("t"), function("is null", field("a"))));
+    assertEqual("source=t ISNULL(a)", filter(relation("t"), function("is null", field("a"))));
   }
 
   @Test
   public void testBooleanIsNotNullFunction() {
     assertEqual(
         "source=t isnotnull(a)", filter(relation("t"), function("is not null", field("a"))));
+    assertEqual(
+        "source=t ISNOTNULL(a)", filter(relation("t"), function("is not null", field("a"))));
   }
 
   /** Todo. search operator should not include functionCall, need to change antlr. */