In service-oriented computing applications, service consumers and providers need to evaluate the trust levels of potential partners before engaging in interactions. The accuracy of trust evaluation greatly affects the success rate of the interaction. Trust evaluation is a challenging problem in open and dynamic environment as there is no central mediator to manage standardized evaluation criteria or reputation records. In this paper, a novel trust model, called the priority-based trust model, is presented. The model derives the trustworthiness of a service provider from designated referees and its historical performance. In addition, consumers can specify their preferred priorities which will affect the result of trust evaluations. The experimental results show that the proposed model has better performance than other trust models, especially in open and dynamic environments.