Abstract: Understanding design rationale is necessary for design reuse.The completeness and coherence of design rationale information got from various documents are difficult to be guaranteed.This article presents an ontologybased method for design rationale information retrieval.The method is based on the knowledge level model of design rationale.Through the query-driven user interface,the questions from which the designer wants to know about the design are dynamically generated and the answers ofthe questions are presented.We build the ontology of design rationale based on related empirical studies and our own case analysis.The implementation ofthe method is illustrated with an example of railway freight car bogie design.Elementary study shows that the method has the advantage of completeness and coherence over conventional information retrieval based on document management and keyword search.It is believed to be one ofthe well-founded techniques of Knowledge Based Design Support System.