The wind turbine gearbox has been increasing the volume and size in order to produce more power. An optimal design for volume of planetary gear is important due to limited space on gearbox. The genetic algorithm is applied for the volume minimization of planetary gear which is used in field. Therefore, the purpose of this paper is studied for optimizing the volume of the planetary gear by the genetic algorithm. In result, it shows that the volume has decreased compared with the existing planetary gear.