Electronic energies of alpha-, beta-, and gamma -FeSi2 were calculated so as to elucidate the possibility of the prediction of phase stability by a quantum-mechanical calculation using a total-energy pseudopotential code, CASTEP. It was properly predicted that the beta -phase is more stable than alpha- and gamma -FeSi,. The effect of the non-stoichiometry of beta -FeSi2 and doping elements (Mn, Cr, Co, and Ni) on the Fermi energy was also discussed.