To further enhance the electrical conductivity of doped ceria, the samarium-doped ceria (SDC)/Al2O3 nanocomposites were prepared through sintering the coprecipitated powders in 1100 degrees C-1300 degrees C. The grain sizes of all composites are less than 100nm and decrease with alumina addition. Besides the main phases of SDC and Al2O3, the SmAlO3 can precipitate in the composites if sintered at higher temperatures or for longer dwell time. The deviations of SDC diffraction peak positions demonstrate the solid solution of alumina into SDC lattice. The total electrical conductivities of the composites increase with alumina content until 30% alumina is added. The SDC/30%Al2O3 presents the higher total conductivity than the pure SDC by about five times. Specifically, the grain interior conductivity generally decreases with the alumina addition while the grain-boundary conductivity increases with that. The introduction of the conductive SDC/Al2O3 interface can contribute to the rise of total conductivity, yet the excessive alumina addition also blocks the oxygen ion conduction. The SmAlO3 precipitation is detrimental to the ion conduction for it consumes part of alumina and leads to the decrement in Sm concentration of SDC grain. Appropriate alumina addition not only enhances the conductivity of SDC but also lowers the material cost.