1990's have been marked by significant changes both in the strategic management field, with the development of competence-based management and the use of the concept of value-creating network, and in the design management field, with the diffusion of matrix-based tools that help to manage the interdependencies between three domains of design projects: product, process, and organization. Few researchers have helped to link these two fields. However, design managers need to use these fields closely together in order to enhance the firm's sustainable competitiveness. Indeed specialists of engineering management have already underlined that design organizations are responsible for the development of lines of products that have to satisfy distinctive stakeholders' requirements. Thus, design organizations strongly contribute to the firm's core competence. In this paper, we outline a method for diagnosing design core competence. We intend to couple strategic management concepts and design management concepts to represent and evaluate design core competence in relation to the product, process, and organizational architectures. The proposed method aims to highlight crucial design organizations, which should require particular managerial attention. The method has been researched and constructed in collaboration with a car design office, and applied in the case of a new robotized gearbox design.