A monitor CNC system based on tow-layer-network was designed. Its controller was based on PC structure, and Windows 2000 operating-system was used, and whose real-time weakness has been solved by RTX. The problems of bad real-time and reliability which were caused by condensed information exchange have been solved by using Profibus-DP and SERCOS bus. Fault monitoring and diagnosis technologies were contact with CNC system designation and a layer-monitoring architecture has been created in prototype machine. Heuristic working in CNC system’s network has been done.