The current article aims to determine the optimum partial gear ratios of three-stage bevel helical gearboxes which can minimize the gearbox cost. Twelve main process parameters are adopted to examine their effects on u1 and u2. A plan of simulation experiment is conducted to serve for finding regression model. It is revealed that the total gearbox ratio parameter, ug, has the biggest effect on both response u1 and u2 when compared to those of others. Moreover, the predicted values given by regression model are significantly consisted of experiments.