Electronic structure of -FeSi 2 and density of states (DOS) of subshells of Fe and Si were calculated, using the self-consistent full-potential linearized augmented plane wave method within the density functional theory. The general gradient approximation was used to treat the exchange and correlation potential. The DOS of -FeSi 2 is mainly composed of that of d-shell electrons of Fe and p-shell electrons of Si. For doped -FeSi 2 , the internal parameters were relaxed, following the damped Newton dynamic schemes. The stable atomic substitution sites of doping atoms were determined by calculating the total energies for Fe 0.9375 Co 0.0625 Si 2 and Fe(Si 0.96875 Al 0.03125 ) 2 . The Fe atom at the Fe II site and the Si atom at the Si I site are found to be preferentially substituted by Co and Al, respectively. This preferential substitution is in good agreement with the results of previous publications. Finally, the electronic structures of relaxed Fe 0.9375 Co 0.0625 Si 2 and Fe(Si 0.96875 Al 0.03125 ) 2 were calculated and analyzed as well. The intrinsic relations between electronic structures of Fe 0.9375 Co 0.0625 Si 2 and Fe(Si 0.96875 Al 0.03125 ) 2 and their respective thermoelectric properties including Seebeck coefficient, electrical conductivity and thermal conductivity are discussed in detail.