The electronic structures of the filled skutterudites CeRu4X12 (X = P, As, Sb) are calculated using the first-principles density functional theory to understand the region close to Fermi energy level (E-F). The effects of the localized electrons present in the sample materials are treated by including the Hubbard's 'U' term in the calculation. The study of the energy band structures suggests the semiconducting nature of CeRu4P12 and CeRu4As12 with the presence of an indirect energy of 0.227 and 0.146 eV, respectively. The addition of Sb atom diminishes the opening of the hybridized energy band-gap above the E-F, giving the metallic nature for CeRu4Sb12. The analysis of the thermal transport properties suggests the high value of Seebeck coefficient with large thermopower at room temperature.