Due to the architecture and characteristics of 3G cellular networks, peer selection algorithms with the idea of traffic localization are not suitable for mobile P2P systems in 3G cellular networks, and load balance on the cells should be taken into account. In this paper, we propose a peer selection algorithm named CFLB for mobile P2P systems in 3G cellular networks. The CFLB algorithm always chooses a peer with the highest available uplink bandwidth from a cell with the lowest traffic load until the number of peers is reached. Simulation results indicate that the CFLB algorithm can achieve excellent load balance on the cells in 3G cellular networks while ensuring favorable peer performance.