Group buying is one dynamic pricing application and it is a mechanism permitting consumers to gather bargaining power in order to get a volume discount or other premiums. In group-buying, the price depends on the number of collected orders. Given a price curve shows the existing orders and current price, the consumer will have some expectation of value and uncertainty for future price. Previous literatures found both the positive participation externality effect and price-drop effect from the field data (Kauffman and Wang, 2001; 2002). Because there are no variable controls from the field study, we can not really know what reasons cause these phenomena in group-buying. Thus, this study will use experimental methodology to the existence of these two effects and understand why they happen. The purpose of this research is to investigate whether positive participation externality effect and price-drop effect exists in group-buying and to explore whether the two effects are related to consumers¡¦ perceived risk and perceived value, whether the intention to join group-buying is influenced by perceived risk and perceived value. The study shows that the two effects really happen in group-buying. Participation externality effect is caused by financial risk, time risk and perceived value. However, price-drop effect is generated from financial risk. Additionally, consumers have higher intentions to join group-buying when they perceive lower financial risk and more perceived value.