Abstract: Design rationale is an explanation and record of product design process, being an important category of design knowledge.The content and representation of design rationale play a vital role in effectively achieving retrieval and reuse of design knowledge.According to the requirements of design knowledge retrieval and reuse, an extended design rationale representation based on ontology is proposed.By analyzing the design knowledge which needs to be retrieved and reused, an extended design rationale representation based on IBIS, combining with FBS model, artifact model and requirements is presented.A design rationale ontology and some semantic rules are constructed accordingly with the purpose of effectively supporting semantic-based retrieval and reuse of design knowledge.The experimental results show that the proposed model is expressive and feasible.