Abstract: Setting up a reliability design model for mechanical transmission system and describing the treatment of correlated variables, a single cylindrical gear reducer was exemplified to illustrate the design of reliability optimization with all of random influencing factors on reducer's operation taken into account. The objective function of reliability design was derived taking variables independent transformation as theoretic basis by which the correlated intensities among individual parts can all be transformed into independent variables from each other, thus making sure the correctness and exactness in calculation. With various constraint conditions defined, a solution was obtained using the penalty function for exterior points to have the advantage of unlimited initial points. Thus, a better result came true through optimization. Finally, the reliability and exactness of this approach was verified by Monte Carlo method and the result showed the similarity between them.