Thermoelectric properties of pure polycrystalline CaMnO 3 ceramics were significantly enhanced by increasing sintering holding time from 1 to 24 h. The Seebeck coefficient values were reduced while the sintering holding time was increased, and the DC electrical conductivity was enhanced from 255 S/m to 1.748 S/m. This same effect was observed in electronic thermal conductivity, increasing from 5x10-3 to 3.5x10-2 W/m K, whereas the lattice thermal conductivity decreased from 5.0 to 4.0 W/m K at 873K. Overall, CaMnO 3 ceramic sintered for 24 h demonstrated the best performance, which presented a Figure-of-Merit value of about 0.03. The grain size of the samples ranged from 2.79 µm to 6.45 µm, due to the sintering holding time, directly influencing the high-temperature thermoelectric properties of CaMnO 3 ceramics.