OPEC occupies an important monopoly position in the international oil market，but it is not a traditional cartel organization，because its behavior mode is not consistent with cartel organizations. As far as OPEC is concerned，its organizational characteristics and behavior patterns leads to a “weak collusion” relationship among its member states. The classification of OPEC as a cartel organization may lead to misunderstanding of its behavior policy analyses. In order to explore the organizational nature of OPEC，this paper puts forward the concept of “quasi-cartel” to analyze it. As an international organization which is constructed by cartel way but not acting by cartel mode，“quasi-cartel” organization has three characteristics：resource monopoly，collective looseness and behavioral difference. At the same time，the behavior pattern of “quasi cartel” organization is closely related to two major factors：① whether there are enough actors with full production capacity in the market；② the choice between share and price among its members. The concept of “quasi-cartel” and the analysis of its characteristics and behavior patterns can better understand OPEC’s collective actions and its related policies，and then it can also play a role in carrying out a relatively more accurate prediction for the organization.