Influence of strongly reducing processing atmosphere on Sr-vacancy Sr1-3x/2Lax/2Smx/2TiO3-δ (x = 0.05, 0.10, 0.15, 0.20, 0.30) ceramics was investigated. The ceramic powders were prepared by the solid-state reaction (SSR) method, and heat treated in 5 % H2/N2 reducing gas at 1573 K for 6 h and 1773 K for 8 h for calcination and sintering processes, respectively. Thermoelectric properties of Sr1-3x/2Lax/2Smx/2TiO3-δ ceramics were evaluated from 573 to 973 K. Their electrical conductivities increased with carrier concentration and also decreased with temperature, indicating metallic behaviour. The Seebeck coefficients showed n-type behaviour and increased with temperature. Additionally, the total thermal conductivities exhibited low values, with a minimum value, 2.67 W/m. K for x = 0.20 ceramics at 973 K. A maximum thermoelectric figure of merit, ZT = 0.30 at 973 K was reached for Sr0.7La0.1Sm0.1TiO3-δ ceramics, which is 20 % higher than the maximum value reported previously for La-Sm electron doped SrTiO3 ceramics.