This paper presents results on mechanical behavior of multilayer piezoelectric stack actuators for use in active member under combined electro-mechanical Loadings. The objective of this study is to investigate the behavior of piezoelectric materials and to determine the properties necessary for design of such actuator systems. Two types of experiments are performed: influences of the preload on characteristics of the stack and dynamic test. The measurements indicate strong dependence of the actuator piezoelectric properties on the electro-mechanical loading conditions. The displacement output is initially enhanced with an increase of the mechanical preload, and the maximum value is obtained at the preload of about 0.4 KN. But much higher preload will cause the decrease of the displacement output. Within low frequency band of about 400 Hz, the displacement magnitude of the stack is nearly constant, and the phase lag increases with the increase of the driving frequency at the applied sine sweep voltage with the amplitude of 150V.