This paper introduces a study on the optimal design of a two-stage bevel helical gearboxe. The objective of the study is to determine the optimal partial gear ratios of the gearbox to achieve the minimum gearbox bottom area. To do that, a simulation experiment was conducted. From the results of the experiment, the influence of main design parameters including the total gearbox ratio, the face width coefficient of bevel gear, the face width coefficient of helical gear, the allowable contact stress of the first and the second stages, and the soutput torque on the optimal gear ratios was investigated. In addition, a regression formula for calculating the optimal gear ratio for the second stage has been proposed.