This article briefly and historically reviews the polices of the Organization of Petroleum Exporting Countries(OPEC) and analyzes the factors that facilitate or hinder OPEC's monopoly in the crude oil market. The industrial concentration ratio is chosen to measure OPEC's monopoly power based on the data from 1986 to 2004. It is concluded that OPEC possesses a long-standing cartel foundation and a rather strong monopoly in the world crude oil market. At the same time, there are unstable factors that influence and even weaken OPEC's monopoly.