diff --git a/tensorrt_llm/_utils.py b/tensorrt_llm/_utils.py
index e733a0331f6..1f37e4a3426 100644
--- a/tensorrt_llm/_utils.py
+++ b/tensorrt_llm/_utils.py
@@ -1016,6 +1016,7 @@ def to_json_str(cls, event):
         return {
             "event_id": event.event_id,
             "data": event_serialize_func(event.data),
+            "window_size": event.window_size
         }
 
     @staticmethod
diff --git a/tests/unittest/llmapi/test_llm_kv_cache_events.py b/tests/unittest/llmapi/test_llm_kv_cache_events.py
index f5efbe2bcf8..8f7fb75c7f4 100644
--- a/tests/unittest/llmapi/test_llm_kv_cache_events.py
+++ b/tests/unittest/llmapi/test_llm_kv_cache_events.py
@@ -71,7 +71,8 @@ def test_kv_cache_event_data_serialization():
     flush_events(kv_cache_manager)
     events = kv_cache_manager.get_latest_events(10)
     serialized_event = KVCacheEventSerializer.serialize(events)
-    assert len(serialized_event) == 1 and serialized_event[0]["event_id"] == 0
+    assert len(serialized_event) == 1 and serialized_event[0][
+        "event_id"] == 0 and serialized_event[0]["window_size"] == 256
     assert serialized_event[0]["data"]["type"] == "created"
     assert len(serialized_event[0]["data"]["num_blocks_per_cache_level"]) == 2