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The oxide-carbonate composite electrolyte material with high ionic conductivity at low temperature has been thought that it can be used to develop LT-SOFC. However, the carbonate composite electrolyte is not easy to make it dense, especially mixing and packing oxide and carbonate to fabricate the composite electrolyte simply. In this article, rare-earth-doped CeO2 (RDC) (R = La, Sm, Gd, and Gd + Y) series samples were prepared by wet ball-milling, then sintered into fully dense and porous oxide bulk at 1500–1600 °C and 1000 °C. Melted carbonate LNCO, composed of Li2CO3 and Na2CO3 at a molar ratio of 1:1, was combined with porous oxide bulk materials using a bath method at 500 °C for 10 h to prepare a dense carbonate-oxide composite electrolyte. The dense oxide-carbonate composite electrolyte always obtains by this fabrication process. Boiling water was used to remove carbonate from these composites. Lattice parameters were obtained through Rietveld refinement, and a calculation procedure for quantifying the composite density was proposed. The quantified composite density results were verified through scanning electron microscopy microstructure observations. The Ce valence in the RDC oxides and RDC-carbonate composite was analyzed by X-ray absorption near edge structure spectroscopy to observe the effects of heat treatment temperature and carbonate on the Ce4+/Ce3+ mixed-valence state in doped CeO2.