The mass, dimension, and size of a gearbox have strong effect on the gearbox cost. Consequently, in order to minimize the cost of a gearbox, the optimization of the partial gear ratios should be found. This study targets to find the optimum partial gear ration of two-stage bevel helical gearboxes base on the objective function of minimizing gearbox cost. Nine main process parameters are adopted to probe their influences on the output response. A plan of simulation experiment is conducted to serve for finding regression model. It is revealed that the total gearbox ratio parameter, ug, has strongest impact on the response u1 when compared to those of others. Besides, the predicted values given by regression model are significantly consisted to experiments