This paper relates the microstructure, crystallographic structure, ferroelectric, and piezoelectric prop-erties of (K0.5Na0.5)NbO3(KNN) ceramics modified with 0.38 mol% K5.4Cu1.3Ta10O29(KCT) and differentamounts of CuO. Results revealed that the addition of KCT and CuO were effective in enhancing thesinterability of KNN. The internal bias field (Eib) increased from 0.3 kV/mm to 0.58 kV/mm at 0.5 mol%CuO–added KNN+KCT ceramics. The increase of Eibcorresponds very well with the observed increase ofthe mechanical quality factor (Qm) from 112 to 2665 for 0.5 mol% CuO. Besides, addition of 0.5 mol% CuOto KNN+KCT resulted in a large increase of the EPR signal, which is related to the increased amount Cu2+and a corresponding increase of the concentration of defect dipoles. This result is in good agreement withthe increased Eiband the resulting hardening behavior.