In this study, performance of gas liquid cylindrical cyclone separators and the effect of changing geometrical parameters of cyclone separators entrance is investigated. The cyclone is simulated with computational fluid dynamic methods. After choosing a suitable mesh grid for the cyclone and checking grid independency, the effect of changing entrance geometry on gas carry under and liquid carry over is investigated. Geometrical parameters, especially inlet geometrical parameters have great effect on optimizing cyclone separators performance. RSM model is used for turbulence simulation of the flow and two phase flow is simulated using Eulerian- Eulerian approach. In this simulation, inlet cross section, inlet angle and inlet height relative to the cyclone bottom part are optimized. Results show that GCU decreases with decreasing nozzle’s inlet angle. An optimum point for GCU was given with changing inlet altitude relative to the bottom of the cyclone and inlet nozzle’s width. An optimum point for LCO was obtained with changing inlet altitude and inlet nozzle width. Increasing inlet angle causes a decrease in LCO. In optimum model, gas carry under decreases significantly and liquid carry over is eliminated.