Abstract: In large scale P2P networks, it is difficult to deal with different type of attacks issued by malicious peers. A novel trust model based on reputation and risk evaluation is proposed in this paper to solve this problem, in which the uncertainty between trust relationships is considered. The risk is quantified using information entropy and further trust degree and uncertainty degree are presented in a uniform form. The simulation results show that the proposed trust model can evidently enhance successful transaction ratio of system and efficiently help peers establish trust relationships in open P2P networks.