The origin of a high mechanical quality in CuO-doped (K, Na)NbO3-based ceramics is addressed by considering the correlations between the lattice positions of Cu ions and the hardening effect in K0.48Na0.52 + xNbO3-0.01CuO ceramics. The Cu ions simultaneously occupy K/Na and Nb sites of these ceramics with x = 0 and 0.02, only occupy the K/Na site of the ceramics with x = −0.02, and mostly form a secondary phase of the ceramics with x = −0.05. The Cu ions lead to the hardening of ceramics with an increase of EC and Qm by only occupying the K/Na site, together with the formation of double hysteresis loops in un-poled compositions. A defect model is proposed to illuminate the origin of a high Qm value, that is, the domain stabilization is dominated by the content of relatively mobile O2− ions in the ceramics, which has a weak bonding with CuK/Na defects.