As the advent of the Internet, B2B negotiation process on the Internet has been given attention from both researchers and practitioners. Therefore, B2B ecommerce decision making will be a challenge for organizations in the foreseeable future. Some literature shows that important issues to reduce uncertainty in the development of long-term relationships among B2B commerce partners. In this sense, this paper proposes a new negotiation support system to incorporate causal relationships of negotiation terms in the process of B2B negotiation, on the basis of a cognitive map. The proposed a CAKES-NEGO (CAusal Knowledge-driven Expert System) suggests that causal relationships of negotiation terms could be explicitly represented by using the cognitive map as knowledge representation vehicle as well as inference engine. Cognitive maps can illustrate causal relationships among the factors describing a given object and/or problem, and it can also describe experts' tacit knowledge about a certain object. A fuzzy cognitive map (FCM) is an extension of a cognitive map with the additional capability of representing feedback through weighted causal links. FCM, a fuzzy signed digraph with causal relationships between concept variables found in a specific application domain, is used for the causal knowledge acquisition. The objectives of this paper are to (1) suggest a fuzzy cognitive mapping based expert system that support decision process of decision makers and (2) apply it to the illustrative examples, which are B2B negotiation problems, to show the validity of our proposed system. In addition, statistical tests proved that the proposed negotiation mechanism could improve decision performance significantly in B2B negotiations.