Design efficiency has significant influence on a company's sustainable competitiveness. Reusing part of the previous product design can undoubtedly improve design efficiency, and thus the need of design knowledge management and reuse is raised. In order to reuse the previous design effectively and efficiently, informal design knowledge, e.g. design experience and design rationale, is required to understand the previous designs completely before reusing them. However, most of the knowledge management activities undertaken in the companies are focused on formal design data and information. Therefore, this research aims to fill in this gap by proposing an integrated knowledge model to guide engineers in capturing useful design knowledge within the design process. The knowledge model integrates four key design elements, namely requirement, function, behaviour, structure, and considers the evolution of the design, to capture informal design knowledge with specific design context. This model has been used in the design and development of a Web-based knowledge management system and preliminary evaluation shows it can achieve effective knowledge capture and reuse.