Although some research and commercial software for NC simulation is available, NC simulation modeling is still not matured. Most of them are based on G&M code. STEP-NC is a new data model for computer numerical control (CNC). It provides rich information for CNC machine tools, i.e. “what to do” based on features rather than “how to do” as for G-code. IDEF0 is a method designed to model the decisions, actions, and activities of an organization or system. It helps to organize and analyze a system and to promote a good communications between the analyst and the customer. So in this paper, from the view of system modeling, function modeling of NC simulation system based on STEP-NC is built by IDEF0 method. As a result, NC simulation system can be realized more efficiently.