The lack of semantic and effective negotiation mechanisms, suppliers and consumers in network manufacturing services are difficult to establish satisfactory service relationships. To solve this problem, an agent-based auto-negotiation mechanism with semantic Web Services was proposed. An agent negotiation model based on the manufacturing services ontology was established. With this model, the major procedures for agent-based auto-negotiation were described, which include how to match the services suppliers and consumers, how to process semantic similarity calculation, and how to auto-negotiation, etc.