Modular design is an engineering concept which will organize and structure complex products or systems into a set of distinct components that could be developed independently and then assembled together. Its objective is to identify independent and standard units which could be used to create a variety of products. In order to successfully implement this concept, the manner in which the modules are selected are critical. By establishing simple interfaces within the modules, the number of interactions are reduced. This would increase the reusability of the modules in different products and allow the cost of the design to be amortized over other projects. In this article, a three phase methodology for modular design of complex products is proposed. In Phase I, the optimum order of the assembly and disassembly of the design concept is identified. In Phase II, concept decomposition is performed. The assembly and subassembly is divided into modules and their interactions are further established. A coding system is used at this stage for classification, retrieval and integration. Finally in Phase III, a complete manufacturability analysis is performed.