Determination of volume or weight of a gearbox is an important issue in preliminary design of power transmission applications. Trial and error procedure or some gear standards information sheets are commonly used in traditional design. The purpose of this paper is to automate preliminary design of gear drives by minimizing volume of gear trains. A stochastic approach Genetic Algorithm (GA) was applied to a parallel axis two stage helical gear trains problem. Static and dynamic penalty functions were introduced to the objective function for handling the design constraints. The results were compared with a deterministic design procedure developed. GA based approach produced quite well results promptly supplying preliminary design parameters of gear drives for different gear ratios to the designer. (C) 2009 Elsevier Ltd. All rights reserved.