We investigated thermoelectric properties of 20-%-Nb-doped SrTiO₃ at high-temperature (300-1000 K) using single-crystalline epitaxial film, polycrystalline film (average grain size ~200 nm) and ceramic (average grain size ~20 μm) samples to clarify the effect of grain boundaries on the thermoelectric performance. Although carrier concentration and Seebeck coefficient of all samples showed no significant differences, Hall mobility of the polycrystalline film was extremely small («10^-1 cm²V^-1s^-1 at 300 K) as compared to those of the epitaxial film and the ceramic sample (~3 cm²V^-1s^-1). However, it drastically increased with increasing temperature and exhibited the value similar to those of the epitaxial film and the ceramic sample above 700 K. The thermoelectric figure of merit for 20%-Nb-doped SrTiO₃ was found to reach 0.35 at 1000 K irrespective of the grain size.