One of drawbacks of solid oxide fuel cells (SOFCs) is high temperature ionic conductivity of their electrolytes. Several researches have been done to find a solution to overcome the problem. One method is to incorporate co-doped cerium oxide (Ceria) with metal salts such as carbonates. In the present work, nanopowders of calcium and samarium co-doped cerium oxide of Ca0.05Sm0.2-xCe0.75O1.9-δ (CSDC 5) were prepared using citrate-nitrate auto combustion method. Phases and crystallite size of the sample were determined by X-ray diffraction (XRD). Field emission scanning electron microscope (FE-SEM) was used for morphological studies. The XRD pattern showed that a single-phase solid solution of calcium and samarium co-doped ceria was obtained. The crystallite size of the particles was 19 nm in diameter as calculated from data obtained through XRD result of (111) peak. FE-SEM images depicted that synthesized CSDC 5 powders were sphere-like and approximately 20 nm in size. Also, it was shown that the sintered CSDC 5 had a dense microstructure. The ionic conductivities of all samples were determined by AC impedance spectroscopy (EIS) in temperature range of 250 – 650ºC. EIS results demonstrated that Ca0.05Sm0.2Ce0.75O1.9- had total conductivity of 0.015 S.cm-1 at 650 ºC. CSDC 5 – (Li,Na)2CO3 (CSDC 5 – LN) nanocomposite electrolyte was synthesized by mixing the lithium and sodium carbonates with CSDC 5 nanopowders. XRD pattern of the composite sample was similar to the patterns of ceria and CSDC 5 which indicates that the carbonate phase was probably amorphous. TG/DTA thermal analysis results demonstrated that the carbonate phase was remained and undecomposed up to temperature of 730ºC. FE-SEM images of the composite powders depicted that CSDC 5 powders were covered by carbonate phase. In addition, the sintered composite showed a porous microstructure using FE-SEM. EIS data proved that above the temperature of 500ºC, total ionic conductivity of CSDC 5 – LN was 0.073 S.cm-1 which was nearly 5 times higher than that of CSDC 5. This implied that using carbonate phase would increase the total ionic conductivity due to multiple ions of Na+, Li+, and CO2-3 would be involved in ionic conduction at the mentioned temperature range. This makes it a serious candidate as electrolyte material for low temperature solid oxide fuel cell (LT-SOFC).