Two-phase horizontal gravity separators are generally used in the petroleum industry for gas and liquid separation. There are several studies in the relevant literature that reports various methods to determine the optimum diameter and length of the separator. Although the diverter plate is used to increase the separation speed, there is not any exact approach for specifying the appropriate location of the diverter plate on the inlet pipe. In the present study, the main volume of the separator is defined for a particular amount of mixture of oil and gas. Effects of the location of the diverter plate and inlet velocity on the separation efficiency are investigated for three locations of the diverter plate (100 mm, 150 mm, 200 mm) and four different inlet velocities (0,25 m/s, 0,5 m/s, 0,75 m/s, 1 m/s) by means of computational fluid dynamics (CFD) method. Two-phase, three-dimensional (3D) and fully turbulent flow simulations reveal that the highest separation efficiency is obtained as 99% when the straight diverter plate is 200 mm far away from the top inlet and the inlet velocity is 0,25 m/s.