This paper mainly studies how to generate test scenarios from UML activity diagram. Firstly, the activity diagram is defined formally, and coverage rules are set down. Then, based on the general thought of dealing with activity diagram with UML activity diagram, a novel method is proposed by adopting semaphore and nested partition to process the concurrent structure of activity diagram for some special elements of activity diagram. It reduces the number of test scenario effectively and also makes automatic generating test case from UML activity diagram become systemic, effective and feasible.