A novel composite material based on mixture of samarium-doped ceria (SDC)-carbonate was studied as electrolyte in low temperature solid oxide fuel cells. The phase and microstructures of composite electrolyte were examined by XRD and SEM. The electrical conductivity was investigated by AC impedance spectroscopy at 400--700℃ in different atmospheres. An abrupt change in the conductivity at about 500℃ indicates that different mechanisms affect transfer in different temperature ranges. The conductivity increases with the carbonate fraction above 500℃. The conductivity in reduce atmosphere is higher than that in oxide atmosphere. An anode-supported fuel cell using SDC-carbonate as electrolyte was fabricated and tested. The result shows that all the composite electrolytes exhibit better performance than pure SDC electrolyte. The electrolyte with 20wt% carbonate can achieve the highest power density of 415mW·cm^-2 and an open circuit voltage of 1.00V at 500℃.