The upper limit of thermoelectric figure of merit, ZT at high-temperature (1000 K) for heavily Nb-doped SrTiO₃ is experimentally clarified using epitaxial films, which were grown on (100)-oriented LaAlO₃ single-crystalline substrates at 700 ^oC by a pulsed-laser deposition method. Carrier concentration, Hall mobility, Seebeck coefficient and thermal conductivity of Nb-doped SrTiO₃ epitaxial films were evaluated at 1000 K according to theoretical analysis. ZT of Nb-doped SrTiO₃ increased with Nb-concentration, and it reached ~ 0.37 (20 % Nb-doped), which is the largest value among n-type oxide semiconductors ever reported.