The effects of CuO on electrical properties of compositionally optimized Li/Ta-modified (Na,K)NbO3 (LT-KNN) piezoceramics were studied to develop LT-KNN with high mechanical quality factor Qm and low dielectric loss tgδ as well as relatively high piezoelectric constant d33 for actuator applications. Both “hard” and “soft” effects were observed due to CuO addition, and the discussion related to high Li doping amount was presented to explain this phenomenon. By increasing CuO addition, the d33 gradually decreases while the Qm increases significantly. Particularly, tgδ at 1kHz can be reduced from ∼2.7% for LT-KNN ceramics to ∼0.8% for CuO-added LT-KNN ceramics. When the CuO addition is lower than 1mol%, both high Qm (∼323) and relatively high d33 (>180pC/N) can be obtained, in addition to a relatively high dielectric constant ɛr∼500.