Formalization of UML models now becomes a requisite action by most of the software designers. UML is semiformal in nature. So it becomes necessary to formalize the UML which would reduce the overall complexity of software design. Today as software becoming more interactive and distributed in nature, the formal syntax and automated verification analysis of behavioral aspect of any model becomes very important in order to reduce overall software development cost and time. UML Activity diagram has become widely acceptable tool for documenting the artifacts related to control flow and complexity of the software system. Here authors proposed the semantics for activity diagram of UML by means of regular expression and its equivalent transition system. UML has now become one of the most widely acceptable standards for visual modeling related to object based software development.