C-Commerce (Collaborative-Commerce) is a new generation business model. In C-Commerce, enterprises collaborate closely with each other on the basis of knowledge sharing. Although previous research have designed knowledge management systems for C-Commerce, existing systems are mostly centralized for intra-organizational knowledge management, which might not manage and share knowledge in a dynamic and inter-organizational C-Commerce environment effectively. In this study, we develop an ontology-based knowledge sharing framework based on semantic web technologies, describe major knowledge sharing processes in the framework, and design a knowledge exchange format as knowledge sharing foundation for the framework. We have implemented these methods and developed a prototype, and experiments are conducted to test the performance of our system. Our work will facilitate knowledge sharing among enterprises in C-Commerce and can be widely applied in e-marketplace, supply chain integration, virtual organizations and other business collaboration across organizations.