The traditional cryptographic mechanisms are not enough for wireless sensor networks (WSNs) when the control of sensor nodes are taken by an interior adversary. Trust manage ment system can solve this problem efficiently, and enhance the security and reliability of WSNs. One of the key problems in trust management system is computing trust values of nodes. However, previous works pay little attention to preserving data privacy during computing trust values. This paper focus on the problem that how to protect the data privacy while computing trust values, and proposes a privacy preserving trust value computation (PPTC) protocol to solve this problem. We develop a new private distributed scalar product protocol based on semi-honest third party to achieve the PPTC protocol. A great advantage of PPTC protocol is that it does not need any encryption or decryption operations, and the computation and communication overheads is low to O(n). What's more, we analyze the correctness and security of the PPTC protocol and prove that the protocol can resist to collusion of up to n -1 users.