According to the traditional methods to design the gears in gearbox, the effect of the volume or noise of gears can not be involved. In order to design better gears, this paper used the simple and effective weighting method and considered the three aspects of minimizing the sum of volume of gears, the bending stress in tooth roots and the working noise to design gears. A mathematical model of optimal design of gears was established, where the contact fatigue strength on tooth surface, bending fatigue strength in tooth roots, tooth number, helix angle, big end modulus, overlap contact ratio, tooth width and so on were set as the constraint conditions and the big end modulus, the tooth width, helix angle of tooth width midpoint, and tooth number of the small bevel gear when transmission ratio is given were chosen as the design variables. Finally an example was solved. The results show that the present method can scheme out the gears of lower noise level.