Trust is one of key factors which influence the development of mobile P2P networks. However, current trust evaluation models are not applicable to mobile P2P networks properly due to some of its characteristics such as heterogeneous nature of the peers, limited-range as well as unreliability of wireless links. In the paper main factors that influence the trust in mobile P2P networks are identified. Based on the analyzed result, we propose a novel trust evaluation model, which helps the networks to operate normally with high probability. Our model does not employ cryptographic approaches or certification mechanisms, so it is light enough to fit well with mobile P2P networks without great overheads. In addition, it also effectively solves the trust problem when no prior interaction history exists, an issue that has not been addressed in many models. The proposed model is shown to be robust in the presence of attackers through simulation.