Knowledge based plastic mold cavity design and manufacturing system is presented. The system architecture is composed of user interface, application sub-system, feature technology, knowledge database and inference engine. The feature technology applied in this system includes feature recognition and feature matching which transforms the geometric model to feature model, and feature mapping which transforms product features into mold cavity features. Knowledge database and inference engine are critical for the knowledge based system. In this system, the knowledge database is divided into rule library and case library and the inference engine is integrated with Rule Based Reasoning (RBR) and Case Based Reasoning (CBR). The result shows that it can effectively integrate the information in the cavity design and manufacturing process and promote the mold design development.