Abstract: The knowledge representation,organization,and knowledge base management are very vital in the Intelligent Computer Aided Design technology based on Expert System.They directly affect design's efficiency and quality.This paper discusses the features of mechanical design process,analyses the usage of knowledge in design,then proposes an unified model to represent all of the design knowledge,including those of design object and design process,an object-oriented and group-based model to organize the elements in the knowledge base.A system named TFMICAD is developed on the basis on these models.It has high efficiency and good adaptability to different design conditions and different design problems,and is easly managed and maintained.