The crystalline phase and thermoelectric properties of sintered Ca₃Co₄O₉/Ca₃Co₂O₆ mixtures prepared by a two-step heat treatment were investigated. Crystal phase and surface morphology were investigated at each step of the heat treatment. The thermoelectric properties of the material and the mixture ratio of Ca₃Co₄O₉ to Ca₃Co₂O₆, as calculated from XRD data, were strongly influenced by the compositional ratio of Co to Ca in the starting material. As the compositional ratio of Co to Ca was increased, the magnitude of the power factor increased and the slope of the power factor decreased with respect to the temperature.