This paper described the technique of predicting the performance of a cyclone oil separator. The oil droplet diameters in refrigerant(R407C)-refrigerating oil were predicted from the experiment by nitrogen-oil. Furthermore, at heoretical analysis model in consideration of both the centrifugal separation and the gravity separation was developed, and the oil separation efficiency was predicted by the Monte Carlo method. The numerical result agreed with the experimental result within an error of 3% in the range of the refrigerant f10w rate 100-600kg/h when the inlet velocity of the oil droplets were about 0.5 times of the refrigerant velocity. In addition, the numerical result showed the separation ratio of the gravity separation and the centrifugal separation to the refrigerant flow rate.