We report the results of a comprehensive study on the structural, electronic, and optical properties of Ga2O3 in its ambient, monoclinic (β) and high-pressure, hexagonal (α) phases in the framework of all-electron density functional theory. In both phases, the conduction band minimum is at the zone center while the valance band maximum is rather flat in the k space. The calculated electron effective mass m∗e∕m0 comes out to be 0.342 and 0.276 for β−Ga2O3 and α−Ga2O3, respectively. The dynamic dielectric function, reflectance, and energy-loss function for both phases are reported for a wide energy range of 0–50eV. The subtle differences in electronic and optical properties can be attributed to the higher symmetry, coordination number of Ga atoms, and packing density in α−Ga2O3 relative to that in β−Ga2O3.