The B3G networks that provide the internetworking techniques of heterogeneous network systems and services are constructed to support various service requirements of mobile users. In this paper, we represent the internetworking scheme of heterogeneous networks systems such like UMTS and WLAN which can be used as access networks, and illustrate the service providing of each network system and the internetworking service based on heterogeneous access networks. UMTS and WLAN can provide each proper service to mobile users, and is included to B3G networks to increase the service usage effectiveness by using complementary service for mobile users.