In this paper, vibration control performance of piezostack active engine mount system for unmanned aero vehicle (UAV) is evaluated via computer simulation. As a first step, the dynamic model of engine mount system which is supported at three points is derived. In the configuration of engine mount system, the inertia type of piezostack based active mount is installed for active vibration control. Then, the vibration level of UAV engine is measured. To attenuate the vibration which is transmitted from engine, a sliding mode controller which is robust to uncertain parameters is designed. Vibration control performances of active engine mount system are evaluated at each mount and center of gravity. Effective Control results are presented in both time and frequency domains.