The effect of MnO2 addition (0–2.0 mol%) on the densification, crystal structure, and ionic conductivity of Ce0.8Sm0.2O1.9 (SDC) was studied. The addition of MnO2 promotes densification, reducing sintering temperature by ~150°C. X-ray diffraction analysis showed that all the samples exhibit a fluorite structure. Impedance spectroscopy measurements indicated that SDC with appropriate ratio of MnO2 addition has higher ionic conductivity and lower activation energy compared with that of SDC. As the addition amount of Mn increases up to 1.0 mol% [(Ce0.8Sm0.2O1.9)0.99+(MnO2)0.01], the sample attains the highest ionic conductivity, about 35% higher than that of SDC at 600°C. In addition, Mn addition has little effect on bulk conductivity, but a marked influence on grain boundary behavior is observed.