Abstract: UML is a widely used modeling language in software engineering, but its main problem is the ambiguity because UML has no formal semantics. A formal execution semantic of UML activity diagram is presented for modeling workflow systems according to the syntax of UML activity diagram and the characteristic of the workflow system. The semantic is based on the clock transition system model. The execution of workflow system is formalized as two processes, clock transition and data transition, executing by turns. Time goes forward in clock transition, and the state of the workflow case transforms in data transition. The semantics is more expressive in concurrency than the semantics based on statechart, more adaptable to the workflow system then Petrinet or process algebra.