Testing concurrent applications is difficult yet important. UML Activity Diagrams are widely used to model concurrent interactions among multiple objects. We present a transformation-based approach to generating scenario-oriented test cases for testing concurrent applications modeled by UML Activity Diagrams. The approach first transforms a UML activity diagram specification into an intermediate representation via a set of transformation rules. From the intermediate representation we then construct a set of test scenarios with respect to the given concurrence coverage criteria. Finally, we derive a set of test cases from the constructed test scenarios. The approach employs transformation to resolve the nonstructural problem with activity diagrams, and can generate test cases on demand to satisfy a given concurrence coverage criteria and hence the number of the resulting test cases is controllable. With the approach, testers can not only earlier schedule the software test process but also better allocate the test resource for testing concurrent applications.