(Na0.5K0.5)NbO3 is doping with ZnO and CuO, respectively, (NKNZx and NKNCx, where x = 01 mol%) using a two-step calcination process (BO method). The X-ray diffraction profiles of NKNZx ceramics show a few secondary phases because Zn2+ ions cannot substitute Nb5+ ions. Moreover, the binding energies of Nb atoms in CuO- and ZnO-doped NKN ceramics are different, suggesting that oxygen vacancies were induced when CuO was doped into NKN ceramics. The mechanical quality factor (Qm) values of the NKNZx ceramics are not as high as those of NKNCx ceramics due to a lack of oxygen vacancies in the former. The internal bias field is used to demonstrate the presence of oxygen vacancies. In ZnO-doped NKN ceramics, the piezoelectric properties only improved slightly because ZnO used as a sintering aid enhanced the densification of samples. A high Qm value is obtained for CuO-doped NKN ceramics mainly due to the formation of oxygen vacancies.