As the second generation of electronic commerce, the framework of agent based commerce is proposed Since the communication messages and solution methods of agents are significantly influenced by the contract type, we have defined the grammar of contracts first. By using this generalized contract grammar, we can express new creative contract types as well as the traditional ones like bids and auctions. The messages in agent based commerce has adopted three layers: Agent Communication Language, Electronic Commerce, and Product Specification layers. For electronic commerce, the agent is regarded as an extension of expert systems with the additional capability of communication control and metaproblem- solving. A prototype of agent based commerce development environment, UNIK-AGENZ is developed to implement the idea. The procedure of message generation and solution method selection is illustrated with the case of computer purchase.