A mechanical system inherently affected by the conditions, factors and parameters of uncertainty. Without including the uncertainty effects in the optimal design procedure, the mechanical system will not be reliable. To include uncertainty effects in an optimum design, there are two different approaches; the robust design optimization and the reliability based design optimization. In robust design optimization process, the main aim is to enhance the system performance by minimizing the variations. In reliability based design optimization process, the target is that the optimal design variables to be in appropriate reliably levels. In this paper, a new model for the reliability based robust design optimization is introduced. First, two new factors that are called “chance – penalty function” and “reliability multiplier” are introduced. Based these new factors, the optimality, the robustness and the reliability as three objective functions are proposed. Then, a combined model for the reliability based robust design optimization (RBRDO) of the mechanical systems under the uncertain conditions is proposed. Finally, the application of the proposed method is demonstrated through design of a tension rod as a case study under uncertain conditions, and the computational results are compared to the obtained results from classical methods.