Abstract: Reuse of design knowledge has become an urgent problem for computer-aided fixture design (CAFD). Ontology is widely used to describe the concept model of information systems at the semantic knowledge level and makes the model reusable. Therefore an ontology based representation approach on fixture design is proposed. Following the structural method of CommonKADS methodology, the fixture design knowledge ontology is established, and represented by the ontology modeling language OWL. The ontology is corresponding to the domain knowledge model of CommonKADS and comprises a detailed specification of the types and relations of the data involved in fixture design process. Assisted by the Protégé system, the basic concepts such as fixturing feature, fixture component, relation and constraint, are defined, amongst which the layered conceptual structure are constructed. Based on the ontology, the concept of rule-type is defined and the rules of CAFD are expressed in semantic Web rule language (SWRL), which improves the interoperability between the rules and the ontology. The facts and rules are captured into the knowledge base with explicit semantic, based on which a prototype system is implemented to infer fixture configuration. Finally examples are presented to demonstrate the application of the ontology. The results show that the ontology is easy for extension and sharing, and it can support logic inference effectively.