Service composition aims to provide the efficient and accurate model of a service, based on which the global service oriented architecture (SOA) can be realized, allowing the value-added services to be generated on the fly. Because of distributed responsibilities, ownership, and control, often, it is not feasible to acquire all information needed for the service composition, thus there might be no guarantee that the service execution has an anticipated effect. In this paper, we are going to extend current Semantic Web Service Description by introducing the concept of "Service Assumption" which allows reasoning with incomplete information. Furthermore, together with the proposed service assumption, a sequence of rules is developed to describe all permitted behaviors in service composition context.