This paper presents the results of experimental analysis and theoretical modeling of piezoelectric stack actuators. The focus of this paper is to understand the behavior of piezoelectric materials under the combined electro-mechanical loadings scenario, and to determine fundamental properties and optimum working conditions. Some parameters, including output displacement, hysteresis, output force and mechanical stiffness, are evaluated under varied pre-stress level and driven voltage values representative of in-service conditions. The measurements indicate strong dependence of the actuator piezoelectric properties and stiffness on the electro-mechanical loading conditions. This research also identified and calculated some parameters of the induced strain actuators electro-mechanical model, which are necessary to performing design optimization to achieve maximum energy transfer and minimum power consume.