In this study, we used conventional ceramics techniques to prepare lead-free ceramics (K0.5Na0.5)NbO3-x mol CuO (KNN- xCu) sintered in air atmosphere and argon atmosphere, respectively, and investigated the corresponding hardening characteristics. The microstructure, P-E hysteresis loops, and X-ray photoelectron spectroscopy results indicate that the Cu2+ ions can play different roles on the hardening mechanism in air atmosphere and argon atmosphere. The Cu2+ ions are considered to replace the Nb5+ ions for forming O2- vacancies, and act as a hardener when sintered in air. However, when sintered in argon atmosphere, a large portion of Cu2+ ions could be reduced into Cu+ ions and then replaced by Na+.