Crystal structure and thermoelectric characteristics of the donor-doped CaMnO3–δ are significantly affected by doping elements and oxygen exchange with ambient air occurring at temperatures above 700 K. To investigate the role of dopants and oxygen nonstoichiometry in thermoelectric phenomena, polycrystalline Ca0.95Bi0.025Y0.025MnO3–δ was synthesized and the obtained temperature dependences of thermal and electronic parameters were studied by DFT calculation method. The electrical conductivity has shown to possess a polaronic character up to ∼700 K with low activation energy. An increase in temperature leads to formation of oxygen vacancies resulting in the appearance of vacancy states inside the band gap and a decrease in its width. The consequence of this is the observed gradual transition to the semiconductor behavior of electrical conductivity explicitly manifested above ∼1000 K. The obtained experimental figure of merit is shown to be almost an order of magnitude less than the corresponding theoretical one for bulk media, the main part of the deviation is related to the electrical conductivity of the ceramic specimen having a polycrystalline nature. We conclude that the thermoelectric properties of the donor-doped manganese oxide can be significantly improved employing bulk monocrystalline media.