diff --git a/spark/src/main/scala/org/apache/comet/serde/QueryPlanSerde.scala b/spark/src/main/scala/org/apache/comet/serde/QueryPlanSerde.scala
index 0bc2c1d3c..45add0ef5 100644
--- a/spark/src/main/scala/org/apache/comet/serde/QueryPlanSerde.scala
+++ b/spark/src/main/scala/org/apache/comet/serde/QueryPlanSerde.scala
@@ -2012,6 +2012,17 @@ object QueryPlanSerde extends Logging with ShimQueryPlanSerde {
     expression
   }
 
+  /**
+   * Returns true if given datatype is supported as a key in DataFusion sort merge join.
+   */
+  def supportedSortMergeJoinEqualType(dataType: DataType): Boolean = dataType match {
+    case _: ByteType | _: ShortType | _: IntegerType | _: LongType | _: FloatType |
+        _: DoubleType | _: StringType | _: DateType | _: TimestampNTZType | _: DecimalType |
+        _: BooleanType =>
+      true
+    case _ => false
+  }
+
   /**
    * Convert a Spark plan operator to a protobuf Comet operator.
    *
@@ -2318,6 +2329,13 @@ object QueryPlanSerde extends Logging with ShimQueryPlanSerde {
             return None
         }
 
+        for (key <- join.leftKeys) {
+          if (!supportedSortMergeJoinEqualType(key.dataType)) {
+            withInfo(op, s"Unsupported join key type ${key.dataType}")
+            return None
+          }
+        }
+
         val leftKeys = join.leftKeys.map(exprToProto(_, join.left.output))
         val rightKeys = join.rightKeys.map(exprToProto(_, join.right.output))
 
diff --git a/spark/src/test/scala/org/apache/comet/exec/CometJoinSuite.scala b/spark/src/test/scala/org/apache/comet/exec/CometJoinSuite.scala
index 54c0baf16..123a2fd70 100644
--- a/spark/src/test/scala/org/apache/comet/exec/CometJoinSuite.scala
+++ b/spark/src/test/scala/org/apache/comet/exec/CometJoinSuite.scala
@@ -40,6 +40,24 @@ class CometJoinSuite extends CometTestBase {
     }
   }
 
+  test("SortMergeJoin with unsupported key type should fall back to Spark") {
+    withSQLConf(
+      SQLConf.SESSION_LOCAL_TIMEZONE.key -> "Asia/Kathmandu",
+      SQLConf.ADAPTIVE_AUTO_BROADCASTJOIN_THRESHOLD.key -> "-1",
+      SQLConf.AUTO_BROADCASTJOIN_THRESHOLD.key -> "-1") {
+      withTable("t1", "t2") {
+        sql("CREATE TABLE t1(name STRING, time TIMESTAMP) USING PARQUET")
+        sql("INSERT OVERWRITE t1 VALUES('a', timestamp'2019-01-01 11:11:11')")
+
+        sql("CREATE TABLE t2(name STRING, time TIMESTAMP) USING PARQUET")
+        sql("INSERT OVERWRITE t2 VALUES('a', timestamp'2019-01-01 11:11:11')")
+
+        val df = sql("SELECT * FROM t1 JOIN t2 ON t1.time = t2.time")
+        checkSparkAnswer(df)
+      }
+    }
+  }
+
   test("Broadcast HashJoin without join filter") {
     assume(isSpark34Plus, "ChunkedByteBuffer is not serializable before Spark 3.4+")
     withSQLConf(