Cellular (3G or 2.5G) networks provide wider service areas and ubiquitous connectivity with low data rates. WLAN networks offer higher data rates and the easy compatibility of wired Internet, but cover smaller areas. 3G and WLAN possess complementary properties. Integrating 3G and WLAN networks may offer subscribers high-speed wireless data services and ubiquitous connectivity. For integrating two different networks, there are several problems should be concerned, such as authentication, billing, quality of service, and seamless roaming between 3G and WLAN networks. In this paper, we address the authentication problem and propose an efficient authentication protocol to let 3G subscribers can connect WLAN with higher data rate. Certainly, the protocol is also suitable for 2.5G. The authentication protocol utilizes a one-time password approach to authenticate subscribers. It can withstand guessing, replay and impersonation attacks. A simulation result is given to validate our protocol.