This paper proposes an activity-based modeling methodology and conceptual framework for an ERP system that oriented to flow manufacturing enterprises. The proposal that activity theory might serve as a model or theory for business process modeling remains an open question. Its purpose is to explore the use of the Activity Theory as a modeling technology to illustrate the proposed methodology. The advantage of this modeling methodology is easy to integrate and extend the especial requirement of the implementation of the middle-sized flow-based enterprises. In this paper, by analyzing the components of an activity and the relationship of the business process modeling, several basic concepts are given and the method of the process modeling is also expressed. Then, a conceptual framework for a flexible integration and extensible ERP system is studied through communicating among different activity units. As a case study, the paper also discusses the process of the information exchange and interface working between the different modules.