Because of the limitations of a singleexpert system or neural network system in fault diagnosis, a new integrative fault diagnosis expert system which combines neural network technique with expert system is proposed. This system is applied in the mechanical breakdown diagnosis of CNC machine. The structure, characteristics and diagnostic methods of the neural network expert system are introduced. The system is verified based on the knowledge of machine failure and sampling data of faults. The result shows that this system has friendly interface and is easy to handle. It raises the level and efficiency of mechanical fault diagnosis of CNC machines.