This study mentions the problem of calculating optimal gear ratio according to the objective function minimal gearbox bottom area. A review of an approach to the optimization of gear ratio is conducted. By using the Design of Experiment technique combined with the Minitab®19 software, we studied the impacts of the design variables on the optimal transmission ratio u2. The results received indicated that the input variables, as well as their interactions, have an important influence on the response u2. Finally, a useful regression model is proposed based on the experiment data and its reliability is confirmed through the results obtained.