The cost of a gearbox depends on many parameters, including its weight. Besides, the weight of the gearbox is highly dependent on the gear ratios of the gearbox. Therefore, to minimize the cost of a gearbox, it is necessary to determine the optimum partial gear ratios. This study is aimed to find the minimum gear ratios to minimize the weight of a two-stage helical gearbox with the second stage double-gear set. To do that, a simulation experiment was designed and performed. Also, nine design factors were chosen for the investigation of their effect on the optimum gear ratios. Besides, regression models to determine the optimum gear ratios were proposed