A collaborative product development environment requires sharing of product information among its participants. Product knowledge models and international standards have been developed for product information sharing. However, their models have limitations in representing their semantics explicitly, so a computer can not understand their semantics properly. It results in lack of information sharing. Recently, ontology gets attentions for automatic knowledge sharing because it can specify the semantics explicitly and logically. In addition, ontology-related standard language, such as web ontology language (OWL), is also proposed. In this paper, we propose an architecture for an ontology-based product knowledge and a product web ontology language (POWL) based on OWL. The architecture consists of three-level ontologies; meta-, generic and particular product ontology. The meta-product ontology is derived from previous top-level ontologies such as SUMO, DOLCE and Guarino’s ontology. The generic product ontology is developed to provide comprehensive knowledge primitives representing product knowledge. A particular product ontology specify knowledge about a specific product such as car, telephone, ship, etc., and it is defined based on the generic product ontology. Meanwhile, POWL has product knowledge primitives defined in the generic product ontology, and it can be transformed to OWL. So users can define the specific product knowledge based on POWL. We implement the tranformation logic with XSLT and demonstrate a POWL usage with an example.