Abstract: Agent organization is an important research aspect of MAS In this paper, the organization is regarded as controlling relations between agents Based on π calculus, a definition of role is proposed, and the process of constructing a concrete agent organization from a frame is given Roles and organization frame are expressed as processes, which defines how an agent joins or departs the organization Compared with Ferber’s work that uses π calculus and the chemical abstract machine to define the formal semantic of organization formation, this work incorporates the role and organization frame explicitly, so it is more general