To improve the performance of case based intelligent stamping die design systems, an approach based on rough set theory is proposed to mine rules from successful design cases. This approach is based on the mapping relation between stamping features and die designs. The mappings from the attributes of a feature to the related die design constitute the design knowledge. The method extracts the rules from the mapping relationship by applying fuzzy classification and computing attribute reduct. The mined rules can be integrated into traditional RBR systems to provide assistance for the design of a new part. Furthermore, these rules can also be used to speed up the case retrieval process of CBR systems by restricting the search space into a subgroup of cases.