The relatively new web service software paradigm involves services which are loosely coupled, highly reusable and flexible. By specifying the workflow of individual services, Web service composition enhances the ability to handle more complex business processes and provides many value-added services. In this article, we propose an extended control flow graph (XCFG) to formally model the workflow of Web service composition specified in BPEL, and corresponding techniques to verify concurrent properties, such as deadlock-free, non-conflict, and link non-redundant. XCFG can model not only the workflow of BPEL but also the synchronization control dependencies among concurrent activities. Meanwhile, each element of XCFG keeps record of related information of corresponding activity in BPEL so as to support further analysis and verification. Experimental study validates the effectiveness and efficiency of the proposed XCFG-based technique.