SrTiO 3 is a promising thermoelectric material for high temperature application of waste heat electrical generation. However, its dimensionless figure of merit ( zT ) is restricted by its high thermal conductivity, which results from the simple perovskite structure and light elements of SrTiO 3 . In this paper, we successfully obtained complex structured bulk SrTiO 3 with a TiO 2 second phase and a porous structure by tuning the doping ratios of the heavy elements La and Nb, resulting in a low thermal conductivity of 1.97 W m −1 K −1 and a high zT value of 0.31 at 1050 K. The first-principles calculation was utilized to calculate the formation energy, effective mass and electron band structure of La-Nb co-doped SrTiO 3 , and the effects of the parameters on the thermoelectric properties were also discussed. We found that the doping ratio of La to Nb changed the electron band structure and controlled the formation of the TiO 2 second phase, which affected the electrical transport and thermal transport properties, respectively. The formation of the in situ TiO 2 second phase was related to the high formation energy derived from the high Nb concentration. These results offer an approach for the design of other thermoelectric materials with low thermal conductivity and a high power factor. SrTiO 3 is a promising thermoelectric material for high temperature application of waste heat electrical generation.