Abstract: This paper describes an expert (FDES) that has been designed to diagnose faults on bearings and gears. FDES is built by the following modules:Feature acquisition, Knowledge acquisition, Knowledge base management, Creating automatically meta-know-ledge, Inference engine, Explanation system and User interface. The system can perform:(a) automatical diagnosis; (b) precise diagnosis. Also, in this system,the method of fuzzy diagnosis is introduced, and the foreward and backward chaining controlled by a "blackboard structure" are used.