Several surface treatments has been applied to the 4140 specimens which are exposed to frictional conditions like gears in order to increase the wear resistance and hardness. Nitration is a common method to improve the hardness of the surfaces. However, the parameters must be chosen wisely. The parameters of temperature, time, flow rate during nitration of SAE 4140 steels were optimized in this study. The ranges for the parameters are selected as 470⁰C -520⁰C -570⁰C, 7h -9h -12h and 6m 3 /h -7.5m 3 /h -9m 3 /h for temperature, time and flow rate respectively. Residual stress, hardness and wear resistance were measured for the specific experiments together with microscopic observations. The nitration process of the threaded part was completed using the optimized parameters. As a result of the study, it was observed that the maximum wear resistance and minimum residual stress values was obtained by the parameters of 470⁰C, 12h, 9m 3 /h.