Design rationale is an important category of design knowledge. Effective reuse of design rationale depends on its successful retrieval. In this paper, an approach to design rationale retrieval using ontology-aided indexing is presented. First, a design rationale ontology is designed based on the extended IBIS-based design rationale representation. Then, an ontology-aided indexing method is given to build indices for design rationale records. Furthermore, three kinds of query modes are developed to support flexible querying. Finally, a prototype system is implemented. The experimental results show the effectiveness of the proposed approach.