Abstract The main idea of group buying is to recruit larger number of orders in order to cut down price. In order words, the larger the number of orders is, the more consumers will join due to the resulted lower price. This positive feedback is as the phenomenon of demand externalities found by Kauffman and Wand (2001, 2002). However, from the other viewpoint, there is a phenomenon of death spiral at the beginning of group buying. It indicates that, due to the few orders in the beginning of group buying, consumers are inclined to wait until the price is acceptable. Unfortunately, it results in that consumers will wait for each other¡¦s joining and therefore, the orders will not increase quickly. In order to solve the problem of death spiral, Lai and Zhuang (2004) proposed three incentive mechanisms, sequential-based, time-based and quantity-based models, on purpose of encouraging consumer to participate in group-buying. The experimental result demonstrated the expected performance of the three incentive mechanisms in terms of removing the phenomenon of death spiral. However, we find these incentive mechanisms didn¡¦t stimulate the phenomenon of demand externalities. Based on previous literature and practical experience, price differentiation usually causes customers¡¦ negative emotion and unfairness cognition. The incentive mechanisms designed for solving the problem of death spiral make consumers pay different price. We wonder if it causes unfairness cognition and therefore could not result in the phenomenon of demand externality. The purpose of this research is to understand whether the incentive mechanisms cause consumers unfairness cognition via experiment. The result shows consumers having opportunity to have extra discounts have higher cognition of price fairness than the consumers without chance to have extra discounts. Further, compared with other incentive mechanisms, sequential-based incentive mechanism makes consumers perceive lower cognition of procedural fairness. Finally, we also found that consumers¡¦ fairness cognition has positive effect on their price satisfaction and purchase intention as well. Keywords: Group buying, Fairness cognition, Consumer behavior, Consumer satisfaction