In order to specify the composition of Web services, WSBPEL was defined as an orchestrating language by an international standards consortium. In this paper, we propose a method to test composite Web service described in BPEL. As a first step, the BPEL specification is transformed into an Intermediate Format (IF) model that is based on timed automata, which enables modeling of timing constraints.We defined a conformance relation between two timed automata(of implementation and specification) and then proposed an algorithm to generate test cases. Test case generation is based on simulation where the exploration is guided by test purposes. The proposed method was implemented in a set of tools which were applied to a common Web service as a case study.