In order to meet the security and efficiency requirements of the access in 3G Network and WLAN ( Wireless Local Area Networks) integrated networks,a localized authentication protocol is proposed based on the roaming ticket. With the aid of the media-independent information service,the mobile user adopting the proposed protocol can find neighbor WLANs,from which one possible target network is chosen in advance. Then,the mobile user requests a roaming ticket for the target network from his home network. As a result,when the mobile user is entering the target network,a fast and safe authentication can be achieved by using the corresponding ticket. Finally,the security of the proposed protocol is verified through the Canetti-Krawczyk model. Simulated results show that the protocol costs less time and needs less message passing.