This article presents an optimization study to find the optimal partial transmission ratios for a mechanical driven system having a chain drive and a worm-helical gearbox. To find the optimum ratios, the system cross section area was selected as the target of the optimization problem. In addition, the effects of the design factors containing the total system ratio, the coefficient for calculating worm diameter coefficient, the wheel face width coefficient, the allowable contact stress of the helical gear set, and the output torque was investigated. To evaluate the influence of these input parameters on the optimum ratios, a simulation experiment was designed and conducted by computer programs. Furthermore, equations for calculating the optimum partial ratios of the systems were suggested. Based on those equations, the determination of optimum ratios can be determined accurately and uncomplicated.