© 2021 Elsevier LtdBismuth germanium oxide (Bi12GeO20) is one of the attractive members of sillenite compounds having fascinating photorefractive characteristics. The electronic, optical and thermodynamic properties of Bi12GeO20 were investigated using density functional theory (DFT) calculations. The experimental and calculated X-ray diffraction patterns were obtained as well-consistent with each other. The lattice constant of the cubic crystalline structure of Bi12GeO20 compound was calculated as 10.304 Å. The electronic band structure and partial density of states plots were reported and contribution of constituent atoms (Bi, Ge, O) to the valence and conduction bands was presented. The band gap energy of the Bi12GeO20 was calculated as 3.20 eV. This wide direct band gap energy provides Bi12GeO20 significant potential in ultraviolet applications. The spectra of real and imaginary components of dielectric function, refractive index, extinction coefficient and absorption coefficient were drawn in the 0−10 eV energy range. Temperature-dependent heat capacity plot indicated the Dulong-Petit limit as 825 J/mol.K. The results of the present study would present worthwhile information to device application areas of Bi12GeO20 compound.