In order to improve a refrigeration system’s energy efficiency, the separation efficiency of its oil separator should be improved. To do so, new oil separator used in a refrigeration system has been developed. The separation efficiency and pressure drop were predicted using CFD. Numerical analysis was performed using FLUENT 14.0. A series of numerical simulation has been conducted while changing the outlet tube length of the oil separator. Depending on the outlet tube length, separation efficiency varies from 98.79 to 98.95%. Considering both of the separation efficiency, outlet tube length of the separator has been designed as 158 mm.