Network-based collaborative product development has become a trend in manufacturing industry, which depends on two key information technology: the Semantic Web and the standard for the exchange of product model data (STEP). EXPRESS is the STEP product data modeling language, and OWL is the standard ontology representing language used in the Semantic Web. Only when the EXPRESS models are converted to OWL models, product information can be exchanged at the Web. The ontology meta-modeling theory was employed to analysis the ontology definition metamodel of EXPRESS and OWL, in order to build up the mapping relationship between them. A formal EXPRESS-to-OWL mapping algorithm is then proposed, represented by abstract syntax.