(Co, Nb, Sm)-doped SnO2-based ceramic with varistor characteristics was successfully fabricated by a conventional sintering of nanosized (Co, Nb, Sm)-doped SnO2 powders prepared by coprecipitation method. Values of α∼38 and EB=1430 V/mm were obtained for the varistor doped with 0.05 mol % Sm2O3, sintered at 1200°C for 1 h. Microstructure development was studied by scanning and transmission electron microscopy, elemental analysis, and X-ray diffraction. The mean grain size of the SnO2-based varistor was 1.75 μm. No other phase besides SnO2 was observed in the varistor doped with 0.05 mol % Sm2O3. The presence of Nb, Co, and Sm inside the SnO2 grains confirmed the formation of solid solution. No segregation of Nb was observed and Nb atoms were homogeneously distributed. The excess amounts of Co were segregated at triple-junctions between SnO2 grains. A Sm-rich region with a typical thickness of 65 nm was observed at the grain boundary of the varistor.