Abstract: The knowledge representation is one of the key problems of mechanical design expert system(MDES). The composite knowledge representation approach to knowledge unit, rule, procedure, vector and list,and the structure and the maintenance means of knowledge base are presented in this paper, on the basis of analysing the design process and the knowledge characteristics of mechanical products,and combining the research of Gear Box Expert System------GBES, GBES is written in Scheme Lisp,It has, run, successfully on an IBM RC/AT computer.