Abstract: This paper presents the overall scheme of GMCADES (General Ma chinery Computer-Aided Design Expert System) which is written in scheme LISP and implemented on IBM-PC microcomputers.The paper coacentrates on clarifying the design and implementation of the inference engine of GMCADES. The inference engine is an important component of GMCADES. It can infer and solve a design problem by forward-inference, backward-inference or mixed-inference. Its structure consists of meta-level inference, object-level inference and blackboard.Well-infering efficiency and the comb ination of inference are the features of the inference engine. In addition, it provi des some important functions,such as data maintenance and manual intervention.