Agent organizational structure evolution is an important aspect in organizational evolution domain. To deal with the problem of agent organizational structure evolution, this paper describes the whole construct of organizational structure based on role, and forms the formalized description of the organizational structure interior characteristic. At the same time, according to the formalized description, mechanism of agent organizational structure evolution based on role is presented. That is, it analyses agent organizational structure evolution process from the three diversification forms which are the role-goals increase, the role-goals decrease and role-relations alteration. In the end, the mechanism provides the reference for the adaptive and dynamic organizational structure.