The vibration isolator suitable for controlling harmonic or random excitation is a key component for the vibration fine-tuned mechanical system. So, various types of vibration-isolators, passive, active, and semi-active, have been adapted to cope with the source of excitations. This paper proposed an optimal design to minimize the vibration response of an emergency electric generator, which generates vibration during operation. The concerned mount module comprised six upper sub-mounts and one basement mass-block and should be effective for both self-excitation as well as basement input. Previous studies addressed the mount optimization of emergency electric generators; the first study proposed a design of excluded the basement mass to reduce the maintenance cost and time of the concrete basement mass and the second one focused on the fine-tuning of mechanical properties of sub-mounts. On the other hand, this study focused on the optimal design of the upper sub-mount. The optimal selection of the upper sub-mount was simulated via three degree-of-freedom model regarding the emergency electric powerplant. Two performance indices regarding the total supporting mount were used to evaluate the control capability of the supported system according to several candidates of the upper sub-mount. It can be found the advancement of the upper sub-mount can be derived for case I as compared to the original one from the simulation consequences. In addition, the effect of the mass volume of the mass-block was also evaluated from the same simulation model.