With the loose coupling and dynamic attributes of Web service, the implementation behavior may be different from original requirement, so itpsilas essential to implement the run-time monitoring. Monitoring analyzes the conformance of a Web service to the requirements. This paper gives a novel monitoring prototype framework. It introduces WS-policy to express the userpsilas monitoring requirement on the services. Then, AOP (aspect oriented programming) technology is used to extend the BPEL execution engine in order to capture run-time information of the service. Finally, monitoring information is analyzed against the EMSC (extended message sequence charts) and METG property specification of that service, reaching the predefined goal of monitoring. To evaluate our approach, we have executed a travel reservation service sample under the monitoring framework. Case study demonstrates the feasibility.