The representation and the process planning of the machine parts are of keyimportance in general CAPP (Computer Aided Process Planning) expert system constructing tool. This paper proposes the knowledge representation model KRM of object--oriented (OO) machine part and process planning according to the theory of similarity among objects to satisfy the requirements of completeness, generalization, logical hierarchical segmentation, dynamic and consistency etc. of the machine part information representation inCAPP system, and present the hybrid inference mechanism of CAPP expert system basedon KRM, including inheritance, multi inheritance, nonmonotonic inheritance and analogical reasoning.