No. 165 Modularity is a new product development strategy in which interfaces shared among components in a given product architecture become specified and standardized to allow for greater substitutability of components across product families. This paper introduces a mathematical model, termed modularization function, for analyzing the degree of modularity in a given product architecture by taking into account the following variables: number of components, number of interfaces, and substitutability factor of a given product architecture. It is assumed that the degree of modularity in a given product architecture is constraint by the composition of its components (number of standard and new-to-the-firm components), interfaces shared among the components, and degree of substitutability. The application of the modularization function is illustrated with two distinct sets of product architectures: Chrysler Jeeps windshield wipers controllers and transmission systems of Schindler elevators. The analysis of the Chrysler case shows that the silent-relay architecture achieved higher opportunities for modularization than the solid-state architecture due to the higher substitutability level and lower new-to-firm component composition. In the Schindler case, on the other hand, the comparative analysis of two different transmission subsystems captures the sensitivity and dynamics of product architecture modularity created by three types of components (standard, neutral, and unique) and two types of interfaces (fundamental and optional). Some managerial implications for both case studies are also discussed based on the application of modularization function.