The response of piezoelectric stack actuator under electric, mechanical, and electro-mechanical loading was investigated. Some parameters were evaluated, including output displacement, hysteresis, output force and mechanical stiffness as a function of mechanical preload and electric field value representative of in-service conditions, to understand the behavior of piezoelectric stack actuator under the combined electro-mechanical loading, and to determine the design-oriented fundamental information. Initially, the output displacement is enhanced with the increase in mechanical preload and the maximum displacement values are obtained when the stacks are preload between 6 - 8 <i>MPa</i>. The hysteresis degree under certain preload has better performance. Stiffness measurements indicate strong dependence on mechanical preload and applied voltage, and the values change significantly depending on the operating conditions. The output force results showed that the external mechanical compress did not have significant influence.