The essence of the getting popular online group-buying activities in the Internet age is that the price of products depends on the quantity of orders. The more the orders are gathered, the lower prices we can get. In addition, the price is always a major concern of consumers in shopping. Thus, the accumulated number of orders is of concern to the initiator and all consumers. In the past, some scholars also found the phenomenon of demand externalities. It indicates that the larger the existing orders are, the more consumers will join the group-buying. On the contrary, it is not easy to attract consumers to join the group-buying. So, if the number of orders increases fast and therefore generates the effect of demand externalities, the performance of group-buying will be improved. Therefore, this research, based the concept of free-options price promotion, explored if the quantity-based price promotion will affect the consumers¡¦ perceived value and intention to join the group-buying. Besides, the research also explored if the product preservation will moderate the effect of quantity-based price promotion on the consumers¡¦ perceived value. In this research, the quantity-based price promotion was manipulated as ¡§Buy five and get one free¡¨ in the experiment. A field experiment was done on NeGoGo group-buying website first. Then, based on the observation of the field experiment, an online experiment was designed and implemented. The result found that consumers have greater perceived value on price and emotion in a group-buying with quantity-based price promotion, but there is no interference effect of product preservation. Furthermore, when consumers have greater perceived value on price and emotion, their intention to join group-buying will be higher. Finally, compared to a group-buying with no price promotion, the average consumers¡¦ purchased quantity of group-buying with quantity-based price promotion is significantly higher. In other words, the quantity-based price promotion can improve the performance of group-buying transaction.