The mechanical vibration often prevents from acheiving higher performance of an electromechanical control system which consists of closed loop electrical part and structural part. A new sensitivity analysis is developed in order to take account the amplitude and phase change of the total control system with respect to the mass or stiffness of the structural part. The modal analysis is used to determine the modal parameters of the structural part and then the servo analysis describes the closed loop characteristics. After identifying these parameters, the amplitude and phase sensitivity can be calculated in the function of frequency. A numerical simulation reveals that the proposed method can predict correct sensitivity. Then a simple electro-mechanical control system is manufactured and the sensitivity analysis is applied to confirm the effectiveness of the method.