Gearbox have the capability of transmitting and adjusting an input motion from a prime mover to drive system. Due to this capability, they are is very widely used and is an indispensable part in industry. In this paper, the analysis of optimum transmission ratio (u2) of helical worm gearbox reducer, based on the objective function of minimizing gearbox mass, are discussed. The resulting optimization problem is solved using the Design of Experiment technique. A plan of a simulation experiment is generated to serve for finding a regression model to calculate the optimum ratio. Analysis of the results received showed that besides the influence of process parameters, their interactions also have significant effects on the response u2.