In this study, the effect of rarefied region in the rotor of the centrifuge was investigated. In a rotor of the centrifuge, the feed inlet is positioned in the rarefied area. The continuum hypothesis is not valid in the rarefied area; therefore, it desires to be analyzed by probabilistic methods like Direct Simulation Monte Carlo (DSMC). In the present study, Computational Fluid Dynamic (CFD) method is used to simulate the continuum area, and the DSMC method is employed in the rarefied area. An implicit coupled density-based scheme was performed for CFD method, and Variable Hard Sphere (VHS) and diffuse model were employed in the DSMC method. Also, the local Knudsen number was defined to determine the interface location between the continuum-rarefied regions (r=0.086 m). The comparison results of pure CFD and CFD-DSMC methods illustrated large differences between the flow properties in the rarefied regions. The results showed that the value of separation power obtained from pure CFD and CFD-DSMC solution is 10.5% different.