Thanks so much. It's solved.
auditpol /set /subcategory:"Network Policy Server" /success:enable /failure:enable
This command enable audit, but local policy overwrite it.
I finally have change the local policy and it works !!!
The success/failure setting can be found at Computer Configuration -> Policies -> Windows Settings -> Security Settings -> Advanced Audit Policy Configuration -> Audit Policies -> Logon/Logoff -> Audit Network Policy Server.
Thanks again.