In this paper, the results of an optimization work to determine the optimal gear ratios of a two-stage helical gearbox to achieve the minimum gearbox bottom area are presented. To do that, a simulation experiment was designed and performed. The main design parameters including the face width coefficient of the firs and the second stages, the allowable contact stress of the first and the second stage, and the output torque were investigated. As a result, the influence of the above main design parameters on the optimal gear ratios has been evaluated. In addition, a regression formula to determine the optimal gear ratio for the first stage was found.