Abstract: The plans made by customer-oriented ERP system for the whole enterprise should be executed based on the actual conditions of plants. But there are often some differences between-plans made by planning floor and actual capabilities of plants because of the limits of ERP itself, dynamic characteristics of plants and a lot of undetermined factors in plants. So Manufacturing Execution Systems were created to solve this problem. They are execution-oriented information systems. MES provides the critical link to fill the gap between ERP and control systems. MES can-execute automatically production plans made by planning floor, extract the correct information for ERP from collected real-time data. MES enables information systems to be interconnected with each other closely and production departments to work in harmony. Consequently, enterprises get maximum profits. In this paper, we explain why MES was created and introduce the development of MES. Based on MESA model, a MES functional model is proposed that comprises two major parts, one related with plan execution and plant management and the other related with data collection and processing. There are 10 functions in the model.