Y2/3Cu3Ti4O12 ceramics were successfully prepared by the conventional solid-state reaction method. Effects of sintering conditions on microstructure, phase structure, and the dielectric properties of Y2/3Cu3Ti4O12 ceramics were investigated in detail. Y2/3Cu3Ti4O12 ceramics sintered at 1010 degrees C for 25 h exhibited a giant dielectric constant (1.10 x 104) and a relatively low dielectric loss (0.033) around room temperature. The samples showed good temperature stability (Delta CT/C25 degrees C=-6.7% 9.5%) in the temperature range from -60 degrees C to 125 degrees C at 10 kHz. It was also found that a new dielectric relaxation III appeared at higher temperatures (>200 degrees C). The complex impedance spectroscopy analysis suggested that Y2/3Cu3Ti4O12 ceramics were electrically heterogeneous, and they consisted of semiconducting grains and insulating grain boundaries, which could be modeled to a first approximation on an equivalent circuit based on two parallel RC elements connected in series. The Cu2+/Cu3+ and Ti3+/Ti4+ aliovalences were observed in Y2/3Cu3Ti4O12 ceramics. The giant permittivity phenomenon could be explained by internal barrier layer capacitance (IBLC) effect.