CuO-doped (1-x)(Na0.2K0.8)NbO3-xBaZrO(3) ceramics (0.0 <= x <= 0.06) were densified at 960 degrees C. The ceramic with x = 0 exhibited a large sprout-shaped strain vs electric-field (S-E) curve and a double polarization vs electric-field (P-E) hysteresis curve, owing to the defect polarization (P-D) developed between Cu2+ ions at Nb5+ sites and oxygen vacancies. The sizes of the S-E and P-E loops decreased with increasing x, owing to the decrease in the number of P(D)s. The ceramic with x = 0.04 displayed small S-E and P-E curves, indicating its small dielectric loss. It exhibited large strain (0.19% at 8.0 kV/mm) at room temperature, which was maintained at 200 degrees C. A similar strain was observed after applying 10(6) cycles of an electric field (3.0 kV/mm). Hence, this specimen exhibited large strain with excellent thermal and fatigue properties. Moreover, the synthesized multilayer actuator using the ceramic with x = 0.04 showed excellent vibrational properties, making it promising for applications in multilayer piezoelectric actuators.