Abstract: The electronic structures of pure CoSb₃, single-filled CoSb₃ and (In, Pr) double-filling CoSb₃ were evaluated by using first principle method. The result showed that (In, Pr) double-filling can further enhance the Fermi level in comparison with In single-filling, giving rise to higher electric conductivity in (In, Pr) double-filling skutterudites. 5p orbitals of In and 6s orbitals of Pr provided the resonant states near conduction band, which was responsible for enhanced Seebeck coefficient of double-filled skutterudites. As a result, (In, Pr) double-filling could increase the power factor due to the enhanced electric conductivity and seebeck coefficient. The calculations agreed well with the obtained experimental data. This work revealed the mechanism to optimize thermoelectric properties of CoSb₃ by double-filling approach.