diff --git a/include/rtthread.h b/include/rtthread.h
index cee58e402b6cfd9286643a7769b0d631eaba5d9e..83be7d1ebc0adc338c918b9a05bab6f799cb58ca 100644
--- a/include/rtthread.h
+++ b/include/rtthread.h
@@ -596,7 +596,7 @@ rt_size_t rt_strnlen(const char *s, rt_ubase_t maxlen);
 #ifdef __ARMCC_VERSION
 /* MDK doesn't have these APIs */
 char* strdup(const char* str);
-size_t strnlen(const char *s, size_t maxlen);
+rt_size_t strnlen(const char *s, rt_size_t maxlen);
 #endif /* __ARMCC_VERSION */
 
 void rt_show_version(void);
diff --git a/src/kservice.c b/src/kservice.c
index cc1dc6e22cbd101765a1370b9452f80cab791ca5..852795309d5faab5291460c0f4faa750607841d1 100644
--- a/src/kservice.c
+++ b/src/kservice.c
@@ -545,7 +545,7 @@ rt_size_t rt_strnlen(const char *s, rt_ubase_t maxlen)
 }
 RTM_EXPORT(rt_strnlen);
 #ifdef __ARMCC_VERSION
-size_t strnlen(const char *s, size_t maxlen) __attribute__((alias("rt_strnlen")));
+rt_size_t strnlen(const char *s, rt_size_t maxlen) __attribute__((alias("rt_strnlen")));
 #endif /* __ARMCC_VERSION */
 #endif /* !defined(RT_KSERVICE_USING_STDLIB) || defined(__ARMCC_VERSION) */