Abstract: In this paper, the fluctuation of the performance function at a given percentile is taken as product robustness index, and the interval reliability of the limit state function bigger than the reliability target is formulated as constraint, the reliability-based robust design single objective optimization problem is developed with the mixture of random design variables and interval variables. To improve the efficiency of probability design optimization problem, the two coupled optimization loops are decomposed into two level structure design optimization, the top level is robust design optimization problem, and the lower level is the reliability analysis problem. Finally, the reliability-based robust design of a reducer is presented to verify the efficiency and accuracy of the proposed method.