Web Service composition is the ability of one business to provide value-added services to its customers through the composition of basic Web services, possibly offered by different companies [12]. Because of distributed responsibilities, ownership and control, it is often not feasible to acquire all information needed for service composition. These characteristics are fundamental to service oriented computing but make it inherently difficult to avoid service conflicts. To reason about and adapt to a changing environment, in this work, we will extend current OWL-S by introducing the concept of service assumptions which allow reasoning with incomplete information. Furthermore, together with the proposed service assumptions, a sequence of rules is proposed to describe all permitted behaviors in service composition context.