diff --git a/include/emqx_mqtt.hrl b/include/emqx_mqtt.hrl
index 35a8a5082..007de4dd1 100644
--- a/include/emqx_mqtt.hrl
+++ b/include/emqx_mqtt.hrl
@@ -78,7 +78,7 @@
 %% Maximum ClientId Length.
 %%--------------------------------------------------------------------
 
--define(MAX_CLIENTID_LEN, 1024).
+-define(MAX_CLIENTID_LEN, 65535).
 
 %%--------------------------------------------------------------------
 %% MQTT Client