In the design of the reducer, the transmission ratio is one of the most important parameters because it is directly related to the volume, size, and cost of the reducer. In this work, we recommend a method to find the optimum gear ratio u1 of the two-stage helical gearbox with the first- stage double gear set to achieve the minimum cost. Whereby, a simulation experiment designed and performed by an optimization program with the goal of minimum cost. Ten input variables were considered to find their influence on the optimal u1. In the end, the proposed regression model is confirmed by determining the deviation between the experiment and response u1 through the residual evaluation distribution.