By controlling the internal pressure in the cyclone separator, we investigated the separation charateristics of Al2O3, Fe2O3 particles with the internal pressure variation. 3-dimensional Langrangian approach was applied for the analysis of the particles separation, and then the minimum cut diameter of the separated particles and the separation rate were calculated through tracking the particle trajectories. The density of the argon gas for transporting particles was decreased corresponding the pressure decrease, consequently, caused the internal pressure drop in the cyclone separator. For that reason the finer particles were separated as the pressure was changed from an atmospheric pressure to an low pressure. Specifically, at 50 torr pressure, Al2O3 particles of the size of about 4 μm and Fe2O3particles of about 3 μm could be separated.