Web service composition is an important technology for agile inter-enterprise application integration. Centric composition engines are widely adopted to enact composite services in many Web service composition research projects. Such a centralized architecture, however, results in problem of scalability, message exchange efficiency and autonomy. A role-based decentralized approach for service composition is proposed in this paper, which partitions the global process model of a composite service into local process models according to the participant roles so as to distribute the control logic of a composite service and corresponding execution load into multiple nodes. An algorithm of generating the local process models is presented in detail with deployment and execution mechanism introduced. Simulation results indicate that this approach can support highly concurrent requests and large volume of data more effectively than the centralized architecture, so it is helpful to improve the scalability of composite services.