To improve the bearing capacity and service life, the gearbox design parameter such as shaft diameter and gear face width can be increased. However, material volume and bending stress should be considered. The purpose of this paper is to improve the gearbox bearing capacity by reducing the bending stress while maintaining material volume increment. To do this the optimization method is needed. Firstly, the shaft diameter and gear face are increased. Secondly, the ANSYS simulation was used to determine the bending stress of the gears. Mathematical model then developed to represent relation between the shaft diameter and gear face as input and material volume and bending stress as the output. Since the mathematical model is non-linear, Artificial Neural Network (ANN) is used to obtained the mathematical model. Finally, Genetic Algorithm (GA) technique of optimization is used to obtain the optimized values. The simulation results clarify that the proposed method was able to achieved optimized value of shaft diameter and face width to obtain the minimal bending stress and material volume of the gearbox.