Using electrical conductivity (σ), Seebeck coefficient (S), thermal expansion, specific heat, and thermal diffusivity measurements in air in the temperature range 300–1200 K, we have determined the thermal conductivity (κ), thermoelectric power factor (S2σ), and thermoelectric figure of merit (ZT = S2σT/κ) of Ca0.5−xSr0.5LuxMnO3−δ (x = 0.05, 0.10, 0.15, 0.20) manganites. The Ca0.45Sr0.5Lu0.05MnO3−δ material has been shown to have the highest S2σ and ZT owing to its high S and low κ.