This paper presents an output power control methodology for wind turbine generator which can decrease frequency deviation of power system using load power estimation. Load power is estimated by a disturbance observer, then output power command for wind turbine generator is determined according to estimated load power. Additionally, the wind turbine generator can also be controlled well during wind turbulence since the power command is determined by considering wind speed conditions. Reduction of frequency deviation of power system by wind turbine generators is achieved using the output power command which is proposed in this paper. In order to verify the proposed method, simulation results with wind turbulence and change of load power are presented.