This paper presents the results of an optimization problem on determining optimal gear ratios of a two-stage bevel helical gearbox to achieve the minimum gearbox cost. To solve this problem, a simulation experiment was performed by building a computer program with the use of the Minitab 19 software to design experiments and analyze experimental results. The influence of the maine design parameters including the total gear ratio, the face width coefficients of the bevel gear set and the helical gear set, the allowable contact stress of the bevel and helical gear sets, the output torque, and the component costs have been evaluated. In particular, the cost of rolling bearing has been taken into account in this study. Moreover, a regression model to find the optimum gear ratio has been proposed.