An oxide is functionally classified into two categories when regarding oxide as thermoelectricity: 1) an oxide itself has thermoelectric properties such as CaMnO₃, homologous series of (ZnO)_m and NaCo₂O₄, and they have been researched since around 1995. 2) there are researches on oxide as an additive to conventional thermoelectric material. We have investigated to approach thermoelectric property from the electronic structures by ab initio calculation. We propose to estimate Seebeck coefficient of oxide by regarding movement of electron between energy levels in the electronic structures. Thereby, it will lead to a new thermoelectric material of oxide. Furthermore, we can improve figure of merit by adding oxide to FeSi₂ due to reducing thermal conductivity in the present study.