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. First, this study proposes an ontology-based knowledge access control model for knowledge sharing in a VE. In the proposed model, user authorizations permitting access to knowledge in a VE are classified into two levels: basic privileges and 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. Based on the proposed knowledge access control model, a knowledge access control policy language model is developed to identify the knowledge access control and sharing rules of a VE and all its enterprise members. The knowledge access control model and the policy language model proposed in this study can facilitate VE Knowledge management and sharing across enterprises, enhance knowledge sharing security and flexibility and regulate knowledge sharing to expeditiously reflect changes in the business environment.