Flettner Rotors are spinning cylinders which produce lift force by utilizing wind power. Today, due to global warming and necessity of reducing emissions, renewable energy resources such as wind energy become more significant than ever before. This study presents the influence of Flettner Rotors on roll motion of ships and the effects of roll motion on performance of Flettner Rotors in beam waves. The calculations are carried out by using XFlow CFD program. The sample rotor has a 4 m diameter, 27 m height and a Thom disc. The calculations are performed with 0, 20, 40 and 60 degrees heel angles to obtain the heeling moment function and driving force function with respect to heeling angle. The results of roll responses of the sample ship with and without Flettner rotors are presented in time and frequency domain comparatively. The results from the calculations show that the lift power performance of Flettner Rotors decreased with increasing of heeling angle. After adding Flettner Rotor contribution on heeling moment function, it is seen that Flettner rotors do not increase the nonlinearity of roll motion. Also, it was found that at large heel angles the driving force becomes almost zero.