Collaborating throughout a product lifecycle via virtual enterprise (VE) is one of the most promising strategies for enhancing global competitiveness. Efficient and secure knowledge sharing is critical to the success of a VE. This study presents a novel approach, model and technology for knowledge access control and sharing across enterprises. First, this study proposes an ontology-based knowledge sharing model and a multiple-layer knowledge representation framework on which a knowledge access control model for knowledge sharing in a VE is proposed. In the proposed model, user authorizations permitting access to knowledge in a VE are classified into two levels: 1) basic privileges and 2) extended privileges. The former are evaluated from four-dimensions, i.e. who, what, when and where, while the latter is determined by considering how three domain ontologies, i.e., product, organization and activity, are related. The knowledge access control model proposed in this study can facilitate VE Knowledge management and sharing across enterprises.