Calcium hydroxide and tricobalt tetroxide were used as starting materials in a Ca:Co molar ratio of 3:4 and sintered at 1073-1373K in a pure oxygen atmosphere. Polycrystalline single-phase Ca3Co4O9 was obtained at all sintering temperatures in oxygen gas, and comparison with a sample sintered in air showed that controlling the oxygen partial pressure during sintering was important. In the sample sintered at 1373K, the Seebeck coefficient was slightly lower than that of the samples sintered at lower temperatures, but the resistivity was decreased by half, so the power factor was doubled.