In modular architectures, Design Structure Matrix (DSM) is used to cluster product components into modules with minimum interfaces externally and maximum internal integration between components. However, DSM is a flat connectivity map that does not capture the layered nature of the product structure. Hierarchical clustering (cladistics) is proposed to automatically build product hierarchical architecture from DSM. The resulting clustering tree represents product architecture while its depth represents its granularity. The optimum granularity level and number of modules are determined, indicating the potential product and process platforms. A case study of automobile body-in-white of 38 components is used to demonstrate the capabilities and superior results quality of the presented technique.