The microstructural properties and the chemical compositions of a 0.65Pb(Zr0.42Ti0.58)O3-0.35Pb(Ni1/3Nb2/3)O3 (0.65PZT58-0.35PNN) ceramic system sintered at 900°C for 4 h with 10 mol% CuO additives were studied using transmission electron microscope (TEM) and energy dispersive spectroscopy (EDS). CuO pockets were found as new microstructural constituents. The liquid phase has formed by the partial melting of CuO additives at the sintering temperature by reacting with Pb element in the matrix. The reaction started at the interfaces and then proceeded into the pocket through the diffusion of the Pb element. The presence of the Pb-rich precipitates during cooling was confirmed by EDS analyses. Cu-rich crystals in the pocket were observed near the boundaries between the matrix grain and the pocket. Rather smaller Pb-Cu-O-contained particle segments were detected around the center of the pocket, demonstrating that the reaction of melting of CuO has occurred with the Pb element which was diffused from the matrix. Due to the existence of the liquid phase, a dense microstructure was achieved during the sintering process and abnormal grain growth occurred in the process.