This paper presents an extended object model for case-based reasoning in product configuration design. In the extended object model a few of methods of knowledge expression are adopted such as constraints, rules, objects etc. On the basis of extended object model, case representation model for case-based reasoning is applied to product configuration design system. The product configuration knowledge can be represented by the extended object. The model can support all the processes of case-based reasoning in product configuration design such as case representation, indexing, retrieving and case revising. The model extends traditional object-oriented model by including the relationship class used to express the relation between the cases, constraints class used in the product configuration knowledge representation, index class used in case retrieving, solution class used in case revise. So the product configuration knowledge that used in the product configuration design can be represented by using this model. In the end a Metering pump product configuration design systems is developed on the basis of the proposed product configuration model to support customized products.