The effects of non-stoichiometry on the microstructure, oxygen vacancies, and piezoelectric properties of (Na0.5K0.5)xNbO3 (NKxN, where x = 0.98, 1.00, 1.01, and 1.02) ceramics doped with sintering aid CuTa2O6 (CT) doping were investigated. X-ray diffraction (XRD) patterns indicated that a secondary phase formed in CT-doped NKxN (NKxNCT) ceramics with x < 1.00 and that a pure phase was obtained with x ≥ 1.00. The grain size of NKxNCT ceramics increased with increasing x value due to the formation of a liquid phase. The internal bias field, activation energy, and Raman analysis for NKxNCT ceramics showed that the number of induced oxygen vacancies increased with decreasing x value. The high mechanical quality factor (Qm) value obtained for NKxNCT ceramics did not correspond to a higher concentration of oxygen vacancies, illustrating that the suitable compensation (excess Na and K) is more important than the concentration of oxygen vacancies to obtain the ceramics with high Qm values. The NKxNCT ceramics with x = 1.01 exhibited excellent piezoelectric properties, with kp and Qm values of 39.9% and 2,070, respectively.