(Na0.5K0.5)NbO3 (NKN) ceramic with 1.5 mol% CuO added (NKNC) was well sintered even at a low temperature of 900°C with the addition of ZnO. Most of the ZnO reacted with the CuO and formed the liquid phase that assisted the densification of the specimens at 900°C. A few Zn2+ ions entered the matrix of the specimens and increased the coercive field (Ec) and Qm values of the specimens. High-piezoelectric properties of kp=0.37, Qm=755, and ɛ3T0=327 were obtained from the NKNC ceramics containing 1.0 mol% ZnO sintered at 900°C for 2 h.