Abstract: Process of MMO server design should focus on the overall structure, communication module and the server robustness. This paper presents MMO servers key technologies and modular organization based on design patterns. The communications module uses IOCP mechanism to guarantee network service concurrency and efficiency. With low coupling, the MMO service module structure is designed with decorator, proxy, observer and other patterns. To ensure the robustness of the server, the service data and service logic are separated with shared memory mechanism and process communication mechanism.