Next generation wireless networks are expected to be heterogeneous, i.e., different networks co-located with different radio access technologies (RATs) work together cooperatively. Load balancing is one of the key techniques in heterogeneous networks which can increase the whole capacity and quality of service (QoS) for the heterogeneous networks. In this paper, a RAT selection algorithm based on fuzzy logic method is introduced to realize load balancing. Voice and data application are considered in the wireless local access network (WLAN) and 3G network based heterogeneous wireless networks. Simulation results show that fuzzy logic algorithm achieves load balancing and thus improves the whole capacity.