This article introduces an optimization study to find the optimal gear ratios for a two-stage helical gearbox with second stage double gear sets to obtain the minimum gearbox bottom area. To do that, a simulation experiment with six main design parameters is conducted. These parameters include the total gearbox ratio, the face width coefficient of both stages, the allowable contact stress of both stages and the output torque. From the results of the experiment, the influence of the main design parameters on the optimal gear ratio of the first stage u1 was evaluated using Minitab software. In addition, a regression formula to calculate u1 has been given.