diff --git a/README.md b/README.md
index f9c7b60..56e4db9 100644
--- a/README.md
+++ b/README.md
@@ -47,7 +47,7 @@ Generate self signed certificate:
 $ cargo r --example generate_self_signed_ca
 ```
 
-When generate success, cert and key will write to files `cert.pem`, `cert.key`.
+When generate success, cert and key will write to files `cert.pem`, `cert.key`, fail if any file is existed.
 
 Important