The knowledge externalization of modular product design is introduced, which discovers and shares the design rules to increase the effectiveness/efficiency of the design process in this paper,. This solution approach involves modelling modular products, formulating their design knowledge, and discovering implicit knowledge into design rules based on the rough set theory. The design rules are generated for improving heat dissipation issues of the set top box (STB), which is perhaps the most attractive device for the home. A web based knowledge management of product design (KMPD) system is also implemented to provide adapted design rules and share design knowledge in distributed environment. The methodology discussed in the paper departs from the previous developments in design knowledge externalization as well as rough set theory application, thus creating a variable approach for product design.