When designing a gear drive, designers should specify tolerances reasonably considering accuracy, cost, and manufacturing capability. In field design, however, designers mostly assign adequate tolerance without correlations between parts and assembly, resulting in iterative design dependent on experts know-how. In order to resolve this, the tolerance design system for a cylindrical gear drive is developed both to support tolerance design automation and to synthesize design processes of part and assembly tolerances. In this research, part tolerances are designed with the databases constructed by ISO, Ks, JIS and bearing catalogue, Assemble tolerance, that is, backlash tolerance is designed by synthesizing part design tolerances stochastically using the formulated assembly relations. This system can include part tolerance and fitting accuracy of shaft adn bearing in practical design. In addition, this system provides field-designers with a synthetic guideline for tolerance design of a gear drive.