The Kozai resonance of Kuiper Belt Objects is explored with a model of circular restricted three-body problem. We use an analytical approach to find out the topological structure of the space of the eccentricity and the argument of perihelion. We find that the objects inside the 2:3 and 1:2 resonances can be inside the Kozai resonance with their arguments of perihelion $\omega$ librating around 90 or 270 degress. It is consistent with the fact that Pluto is in both the 2:3 resonance and the Kozai resonance. In addition, the objects outside the mean motion resonance are also found to be in the Kozai resonance. We discover that the stable equilibrium points of $\omega$ are at 90 and 270 degrees, but not 0 and 180 degrees as shown in the work of Thomas & Morbidelli (1996). To verify our results, numerical experiments are worked out with different longitudes of the node and mean anomalies. These experiments confirm our analytical results. Some test particles are found in the Kozai resonance around 90 degrees lasting for 1 billion years, and the libration amplitudes of their arguments of perihelion are quite small. Combining with the work of Kozai (1962), we conclude that the Kozai resonance protects a small body from close encounter with an internal perturber around the perihelion or with an external perturber around the aphelion.