The microstructure, thermal expansion, mechanical property, and ionic conductivity of samaria‐doped ceria (SDC) prepared by coprecipitation are investigated in this chapter. The results revealed that the average particle size ranged from 10.9 + 0.4 to 13.5 ±0.5 nm, crystallite dimension varied from 8.6 + 0.3 to 10.7 + 0.4 nm, and the specific surface area distribution ranged from 62.6 + 1.8 to 76.7 + 2.2 m/g for SDC powders prepared by coprecipitation. For SDC ceramics sintered at 1500°C for 5 h, the bulk density was over 95% of the theoretical density the maximum ionic conductivity, σ800 °c = (22.3 + 1.14) × 10−3 S/cm with minimum activation energy, Ea = 0.89 + 0.02 eV, was found in the Ceo.80Smo.20O1.90 ceramic. When the soaking time was increased, the microhardness of Ceo.8Smo.2O1.9 ceramic increased, the toughness slightly decreased, which was related to grain growth with the soaking time.