In this study, 1.0 mol% CuO-added (NaxK1-x)NbO3 (CNxK1-xN) ceramics with 0.0 x 0.5 were sintered at a relatively low temperature of 960 degrees C. A large Q(m) value (1750) was obtained for the specimens with x 0.2 owing to the hardening effect of Cu2+ ions, indicating that a large number of Cu2+ ions had replaced Nb5+ ions in these ceramics. The double polarization versus electric field (P-E) hysteresis loop was observed for the specimens with x 0.2, and was explained by the defect polarization (P-d) formed between Cu2+ ions and O2- vacancies. These specimens also exhibited a sprout-shaped strain versus electric field (S-E) curves with a large strain of 0.14%-0.15% at 8.0 kV/mm because of the internal bias field produced by P-d. For the CN0.2K0.8N ceramic, a double P-E hysteresis loop was maintained at 125 degrees C without remnant polarization. Moreover, this ceramic showed a large strain of 0.175% after 10(4) cycles when a high electric field of 8.0 kV/mm was continuously applied to the specimens during each cycles. Therefore, CN0.2K0.8N is a useful lead-free piezoelectric ceramic for multilayer actuators.