0.65Pb(Zr1-xTix)O-3-0.35Pb(Ni0.33Nb0.67)O-3 (0.65PZTx-0.35 PNN) ceramics exhibited a morphotropic phase boundary at approximately x = 0.58-0.59. The 0.65PZT58-0.35PNN ceramics sintered at 1200 degrees C exhibited the high piezoelectric properties of d(33) = 605pC/N, k(p) = 0.61, and epsilon(T)(3)/epsilon(0) = 3600. The sintering temperature of the PZT58-PNN ceramics decreased from 1200 to 850 degrees 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 degrees C and was not altered by the addition of CuO. The d(33), k(p), and epsilon(T)(3)/epsilon(0) 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 degrees C exhibited the high piezoelectric properties of d(33) = 620 pC/N, k(p) = 0.64, and epsilon(T)(3)/epsilon(0) = 3750. Moreover, the reaction between the Ag electrode and the Cu-added 0.65PZT58-0.35PNN ceramic sintered at 900 degrees C was negligible.