The vibration isolation performance of vehicle powertrain mounting system is mostly determined by the three-directional stiffness of each mount block. Because of the manufacturing tolerance and the coupling effect, the stiffness of mounts cannot be maintained stable. The purpose of this study was to find out the way to optimize the stiffness of mounts via the design of experiments (DOE).According to the DOE process, a full factorial design was implemented. The z-direction stiffness of three mount blocks in the mounting system was selected as the three analysis factors. The maximum and the minimum stiffness of each mount block within the manufacturing tolerance were selected as the two levels. The measured vibration of vehicle body under certain loading case was selected as the response factor. After eight times of experiment, the DOE parameters were analyzed with statistical methods.The final DOE results showed that the vibration of vehicle could be expressed by a linear equation, the parameters in which are the stiffness of mount blocks. And the analysis of variance indicated that the vibration of vehicle is mostly affected by the stiffness of engine mount block.