Increasing developments of systems using web applications, SOA (Service Oriented Architecture) based development becomes the focus of attention. However, in SOA based development, it is difficult to understand behaviors of system, which is integrated with many web applications. Therefore, it is necessary to simulate the entire behavior in early stage. A formal semantics for UML activity diagrams has been proposed in [1], where an activity diagram is mapped into a hypergraph called an activity hypergraph, and the semantics is defined on the hypergraphs. In this paper, we propose a mapping from BPEL4WS/UML descriptions into activity hypergraphs, and enhance the semantics to simulate entire systems.