Abstract: In this paper we introduce a practical computer-aided design （CAD） system towards mechanical parts design on the basis of our researches on product modelling systems at the Institute of Artificial Intelligence in Zhejiang University. Firstly the methodology to develop practical and commercial CAD systems from our practice of designing real systems is presented, then the system architecture and general functions of the system are introduced, and lastly the structure and implementation of some important modules of the system are discussed.