Piezo stack actuators have been widely adopted for the actuation of flexure hinge based micro-motion compliant mechanisms (or sometimes named stages). When fitting a Piezo stack actuators into a compliant mechanism, normally no adhesive is applied but tightly fitted by pushing it into the slot (desired position). As the slot is normally designed to be slightly smaller than the Piezo stack actuator or a tightening mechanism could be used instead, by fitting the Piezo stack actuator into position, the compliant mechanism is therefore slightly deformed and pre-load is resulted on the Piezo stack actuator. This paper presents the original investigation about the level of changes of compliant mechanism stiffness caused by tightly fitting Piezo stack actuator into the mechanism. Finite Element Analysis simulations were conducted on a particular one DOF flexure hinge based Compliant Mechanism and the results demonstrate the stiffness of a compliant mechanism would certainly be changed and the extend of change is dependent on how the Piezo stack actuator is fitted into the mechanism.