Abstract: Current product modeling technology is deficient in knowledge integration and sharing during the whole product lifecycle. In this paper we propose an ontology-based formal semantic expression framework, which is compatible with ISO STEP. In order to achieve effective product knowledge representation and reasoning, we introduced a description logic language, named ALCNRP(D), as the building tool to specify product ontology. We also analyzed the semantic characteristics of EXPRESS constructors and provided corresponding way of expression in description logic language to abstract and describe the ontological concepts contained in EXPRESS SCHEMA and finally, establish the ontology level of product data.