Polycrystalline samples were synthesized by a conventional solid-state reaction. As starting materials, Co(3)O(4) and Ca(OH)(2) were mixed in a molar ratio of Ca:Co=3:4 and sintered at 1073-1373 K. The compound phases and thermoelectric properties of the resulting products were dependent on the sintering temperature. X-ray diffraction analysis showed that the Ca(3)Co(4)O(9) compound was stable up to 1193 K and that Ca(3)Co(2)O(6) was formed at 1273 K. At 1373 K, the sample decomposed into cobalt oxide and calcium oxide. A maximum power factor of about 100 mu W/(K2 center dot m) at 873 K was obtained for the Ca(3)Co(4)O(9) phase sintered at 1193 K.