Abstract The effect of Sr addition (0–10 mol%) on the densification, crystal structure, ionic conductivity, thermal expansion, and flexural strength of Ce 0.8Sm 0.2O 1.9 (SDC) was studied. Sr addition promotes densification and reduces sintering temperature by ∼100 °C. X-ray diffraction analysis showed that all the samples exhibit a fluorite structure. Impedance spectroscopy measurements indicated that SDC incorporated with an appropriate ratio of Sr (4–10 mol%) has higher ionic conductivity and lower activation energy compared with those of SDC. As the added amount of Sr increases by up to 6 mol% (0.94SDC-0.06Sr), the sample attains the highest ionic conductivity, about 30% higher than that of SDC at 700 °C. Moreover, flexural strength increases with Sr content, with the addition of 10 mol% Sr increasing the flexural strength of SDC from 97 to 160 MPa. In addition, thermal expansion is linear for all the samples. The addition of Sr does not appreciably change thermal expansion coefficient.