Abstract: In large scale P2P networks, it is difficult to establish the trust relationship between peers as it is less likely that repeat interactions will occur with the same peer. So the network is vulnerable to malicious peers. A group based hierarchical reputation model for P2P networks is proposed in this paper to solve this problem. In this model, the trust relationship is classified into three tiers: the trust relationship between groups, the trust relationship between groups and peers, and the trust relationship between peers. A peer evaluates the trust of the given peer by its local trust information or the reference from the group it belonging to. The simulation and analyses show that the proposed reputation model can distinguish good peers and make them happy with the high ratio of the success query.