In this study, composition of (Na0.5K0.5)NbO3 + x mol% CuO ceramics (NKNCx, where x = 01 mol%) were separately prepared using the B-site oxide precursor method (BO method) and the conventional mixed oxide method (MO method). The diffracted angles of the NKNCx ceramics decreased as the concentrations of CuO dopants increased, because Nb5+ ions were replaced by Cu2+ ions, leading to the formation of oxygen vacancies. The microstructures of NKNCx ceramics prepared using the BO method were more homogeneous microstructures than that of ceramics prepared using the MO method. The mechanical quality factor (Qm) and dielectric constant (e33T/e0) of NKNCx ceramics prepared using the BO method were therefore by 21% and 25% better, respectively. The activation energy and internal bias field were measured and calculated to confirm the presence of oxygen vacancies. A low activation energy corresponds to a high Qm value. NKN ceramics with 0.75 mol% CuO doping prepared using the BO method had an activation energy and internal bias field of 0.95 eV and 3.5 kV/cm, respectively, at x = 0.75 mol%. The bulk density, kp, Qm, e33T/e0, and dielectric loss of NKNCx ceramics with this composition were 4.488 g/cm3, 41.5%, 2100, 280, and 0.15%, respectively.