This chapter designs a multi-agent argumentation system for e-commerce. This system applies Semantic Web technology to facilitate agents to share ontologies and describe their own mental states and arguments. All arguments are connected by attacking relations and can be proved or defeated via a dialectical game. In this system, buyer and seller agents can understand arguments and argue over product attributes. This system can help buyers to delegate their buyer agents to search products that exactly match their needs, and help sellers to delegate seller agents to present products and persuade buyer agents into believing that the products can satisfy the buyers’ needs.