Na0.5Sm0.5Cu3Ti4O12 (NSCTO) ceramics have been prepared by reactive sintering of amorphous powder.Spark plasma sintering (SPS) for 10 min at 1025 ℃ and conventional sintering (CS) for 10 h at 1090 ℃ have been employed.X-ray diffraction measurements confirmed the pure CCTO-like phase for SPS and CS NSCTO ceramics.The SPS ceramic showed an average grain size of 500 nm,which is much smaller than that of the CS (～ 5 μm) sample.The impedance spectroscopy measurements revealed an electrically inhomogeneous structure in the prepared ceramics.While the resistivities of grains of both ceramic samples were in the same order of magnitude,the resistivity of grain-boundaries of the CS ceramic was three orders of magnitude greater than that of the SPS ceramic.Both of the samples showed giant dielectric constant (＞ 103) over wide ranges of temperatures and frequencies.Nevertheless,the room-temperature dielectric loss of the SPS NSCTO (3.2 at 1.1 kHz) ceramic sample was higher than that of the CS NSCTO (0.08 at 1.1 kHz) ceramic sample due to the reduced grain-boundary resistivity of the former.Two dielectric relaxations were detected for each sample and attributed to the relaxations in grains and grain-boundaries.The dielectric behavior of the SPS and CS NSCTO ceramics could be interpreted in terms of the inteal barrier layer capacitor (IBLC) model.