. Excitation force produced during gear mating of power transmission system induces vibration to the gearbox. Resonance occurs when excitation force frequency coincides to one of the natural frequencies of the gearbox which will bring not only sudden and unexpected vibration but also potential destruction to the gearbox. Natural frequency is determined by mass and stiffness of gearbox. These two determiners rely on sizes of the parts of the gearbox. That means natural frequency can be shifted by varying the sizes to prevent resonance. To demonstrate this, in this paper, the two stage parallel shaft gearbox modeled by shell-solid elements has been considered. The modal analysis and the real eigenvalue sensitivity analysis have been performed, in MSC. NASTRAN. The sensitivity coefficient and frequency spectrum at particular thickness of gearbox part show the effect on shifting the mode frequency.