Abstract: The overall structure, control strategies and inference engine of me chanical design expert system i.s one of the key problems to solve complex mechanical design problems efficiently. In this paper, those .problems are discussed, combining the research of Gear Box Expert System-GBES, the modularized over all structure of GBES, and the control strategies, inference engine; blackboard control method, inferential chain, means-ends analysis, forward and backward reasoning, are presented.