In this paper, the results of an optimization study to determine the optimal gear ratio for a two-speed helical gearbox with first-stage double gear sets are presented. The objective function of the problem is the minimum bottom area of the gearbox. To solve this problem, a simulation experiment was performed. In particular, the influence of six main design parameters including the total gear ratio of the gearbox, the wheel face width coefficients of both stages, the allowable contact stress of the first and the second stages and the output torque to the optimal gear ratio of the first stage u1 has been evaluated. In addition, a regression formula to find the optimal gear ratio u1 was given.