A traditional solid-state sintering method was used to fabricate 0.55Pb(Ni1/3Nb2/3)O-3-0.45Pb(Zr0.3Ti0.7)O-3 (0.55PNN-0.45PZT) relaxor-ferroelectric ceramics with CuO as sintering aid. The influence of xCuO additions (x=0.0, 0.2, 0.4, 0.6, 0.8 wt.%) on the microstructure and electrical properties of the 0.55PNN-0.45PZT ceramics was investigated. X-ray diffraction patterns demonstrated that all specimens formed a typical perovskite structure with pseudo-cubic phase. Cu2+ ions entered into the B sites in ABO(3) structure for the PNN-PZT solid solution ceramics. The additions of CuO improved the sinterability of the 0.55PNN-0.45PZT ceramics. The grain size and density of these specimens increased slightly with increasing amounts of CuO addition. The specimen with 0.4 wt.% CuO addition achieves the maximum value of density rho similar to 8108 kg/m(3). Meanwhile, the piezoelectric, dielectric and ferroelectric properties also enhanced significantly with increasing amount of CuO addition. Excellent properties, d(33)similar to 777 pC/N, k(p)similar to 55%, epsilon(r)similar to 7365, tan delta similar to 2.9%, p(r)similar to 18.86 mu C/cm(2), and E-c similar to 3.52 kV/cm were obtained for the 0.55PNN-0.45PZT ceramics with 0.4 wt.% CuO addition sintered at 1125 degrees C for 2 h.