Product family design facilitates mass customization by allowing highly differentiated products to be developed around a platform while targeting products to distinct market segments. Therefore, effective platforming of products is a cost-effective way to achieve mass customization The objective in this research is to develop a Strategic Module-based Platform Design Method (SMPDM) to determine a platform design strategy to support product family design in a dynamic and uncertain environment. Ontologies are used to represent products and enable sharing and reuse of design information. Data mining techniques are used to identify a platform and modules by utilizing design information stored in a large database or repository. To determine a platform for family design in dynamic and uncertain market environments, the SMPDM uses agent-based decision-making, involving a market-based negotiation mechanism and a game theoretic approach based on module-based platform concepts and a mathematical model. To demonstrate and validate the usefulness of the proposed method, it is applied to a family of power tools and tested in multiple scenario-based experiments. The SMPDM provides an optimal platform design strategy that can be adapted to various dynamic and uncertain market environments. Therefore, the SMPDM can help develop design strategies to manage and create a cost-effective variety of products based on a platform in support of mass customization.