This paper discusses the incorporation of modularization into mechanical designs. The research uses a definition of modularity that incorporates the potential of modularity based not only on the form/function structure of a product but also life-cycle processes such as manufacture, assembly, service, and recycling. Modularization, due to the functional independence it creates, has been called the goal of good design. Industry has made an effort to modularize products to be flexible to the needs of end users. In addition, some modules are created with some aspects of assembly in mind. Life-cycle modularity entails maintaining independence between components and all life-cycle processes in different modules, encouraging similarity in all components and processes in a module, and maintaining interchangeability between modules. In this paper, the definition of product modularity is given. A measure of relative modularity and a modular design methodology are developed that encourage modularity, prevent a cascade of product design changes due to changes in life-cycle processes, and support agile reaction to changes in life-cycle processes. A short example is used to clarify the work.