The mount stiffness of a powertrain mounting system (PMS) generally fluctuates around its nominal design value due to measurement inaccuracy, processing and installation errors, and material aging. To improve the robustness of the frequency allocation and decoupling layout of a PMS, an interval optimization model is presented, in which interval number is used to describe the uncertainty of the mount stiffness, and interval reliability and interval possibility indices are used to characterize the robustness of the decoupling layout and frequency allocation, respectively. Then a six-sigma robust optimization model is constructed in which the mount stiffness is assumed to obey uniform distribution. The interval optimization and six-sigma optimization approaches are used to maximize the decoupling ratios of a general car PMS under robustness requirements of frequency allocation and decoupling layout. Optimization results show that both the interval optimization and the six-sigma optimization can obtain satisfactory robust design, and the optimization results of the two optimization methods have good consistency.