BPEL (Business Process Execution Language) as a de-facto standard for web service orchestration has drawn particularly attention from researchers and industries. BPEL is a semi-formal flow language with complex features such as concurrency and hierarchy. To test a model thoroughly, we need to cover different execution scenarios. As is well known, it is tedious, time consuming, and error prone to design test cases manually, especially for complex modelling languages. Hence, it is desirable to apply existing model-based-testing techniques in the domain of web services. We proposed WSA (Web Service Automata) to be the operational semantics for BPEL. Based on WSA, we propose a model checking based test case generation framework for BPEL. The SPIN and NuSMV model checkers are used as the test generation engine, and the conventional structural test coverage criteria are encoded into LTL and CTL temporal logic. State coverage and transition coverage are used for BPEL control flow testing, and all-du-path coverage is used for BPEL data flow testing. Two levels of test cases can be generated to test whether the implementation of web services conforms to the BPEL behaviour and WSDL interface models. The generated test cases are executed on the JUnit test execution engine.