UML has become de facto standards for object-oriented software development. It provides rich notations for representing, analyzing architecture and behaviors of systems. Among these notations, UML activity diagram is well-known for describing system?s dynamic behaviors. One of its main purposes is to model software processes and business processes, and represent control flows among activities. As a semi-formal notation, UML activity diagram has not publicly agreed formal semantics. In this paper, the alternative approach of using π - calculus to formalize UML activity diagrams is presented. Therefore, activity models can be of rich process semantics. Its main advantages have: (1) According to weak bisimulation in π - calculus two process models can be checked for equivalence, thus facilitating the optimization of business processes or software processes. (2) Process model can be verified whether it satisfies certain properties, such as safety and liveness, by means of π - calculus analytical tools. (3) Correct criterions for process model are expressed with modal M? - calculus, such as the correct termination of processes. Therefore, process model can be automatically verified with the help of π - calculus analytical tools.