Abstract: Monolithic multilayer piezoelectric actuator was tested by two nondestructive methods, scanning electron microscopy (SEM) and scanning electron-acoustic microscopy (SEAM). The SEAM image of the ceramics/internal electrode interface region was found much wider than the SEM image. Based on the SEAM imaging mechanism, the results of EDS analysis and the layer structure of the actuator, this effect was interpretted as being resulted from the large thermal internal stress at the interface between ceramics and the internal electrode. The thermal internal stress is induced by the different thermal contraction during preparation of actuator using thermal pressure and cofiring process, and is the cause of delamination for some actuators during cofiring or under an applied voltage in application.