Abstract: In this paper,the development history of wireframe model,solid model and feature-based model is reviewed.In order to achieve high efficiency,a CAD system should support the conceptual and layout design stage of product.First,the paper uses mould design as an example to show the building up of structure-oriented function model with the object-oriented methodology.Based on this model,the mapping relationships between function and feature,function and part are discussed.Finally,a formulation representation scheme which is called position CSG tree is proposed.The position CSG tree describes the position relationship among components of product.With the above analysis,the current feature modelling technology,solid modelling technology as well as data base,primitives library of specific domain can be used to design the product details effciently.