This work applied three-dimensional and volume of fluid (VOF) model of computational fluid dynamic (CFD) simulation to demonstrate the movement of liquid-liquid interface in a primary oil-water separator. Basic sediment and water as well as salt removal had been used as criteria for the separator efficiency, experimentally. Water removal from organic phase was considered as the efficiency using CFD. Simulation was performed for different water levels, and the way this parameter can influence the efficiency was studied. Experimental and CFD simulation results were in relative agreement and showed that the optimum water level is 58 and 50% of separator's diameter, respectively.