0.65Pb(Zr1−xTix)O3–0.35Pb(Ni0.33Nb0.67)O3 (0.65PZTx–0.35PNN) ceramics exhibited a morphotropic phase boundary at approximately x=0.58–0.59. The 0.65PZT58–0.35PNN ceramics sintered at 1200°C exhibited the high piezoelectric properties of d33=605pC/N, kp=0.61, and ɛ3T/ɛ0=3600. The sintering temperature of the PZT58–PNN ceramics decreased from 1200 to 850°C with the addition of a small amount of CuO. The CuO reacted with the PbO and formed a liquid phase during sintering, which assisted in the densification of the specimens. Most of the Cu2+ ions existed in the CuO second phase, thereby preventing any possible hardening effect due to the Cu2+ ions. The Curie temperature of the 0.65PZT58–0.35PNN ceramic was approximately 200°C and was not altered by the addition of CuO. The d33, kp, and ɛ3T/ɛ0 values of the 0.65PZT58–0.35PNN ceramics were considerably increased with the addition of CuO and similar results were also observed for the 0.65PZT59–0.35PNN ceramics. In particular, 1.0 mol% CuO-added 0.65PZT58–0.35PNN ceramics sintered at 900°C exhibited the high piezoelectric properties of d33=620 pC/N, kp=0.64, and ɛ3T/ɛ0=3750. Moreover, the reaction between the Ag electrode and the Cu-added 0.65PZT58–0.35PNN ceramic sintered at 900°C was negligible.