Workflow model is short of a kind of formal mathematics model which support process definition and process analysis. The -calculus is a kind of mobile process algebra which can be used to model concurrent and dynamic systems. Firstly, proposes a modeling method for workflow based on the -calculus. Furthermore, after comparing and analyzing a variety of modeling tools, makes a formal definition for workflow structure by using -calculus, and clarifies the expression of activities and dependency relations in -calculus. The method is a complete formal method, which has a strong capability in semantics expression and is easy to implement, reason and simulate workflows.