Maximizing the thermoelectric figure-of-merit (ZT) is a challenge owing to the conflicting combination of material properties. We explored simultaneously enhancing the power factor and reducing the thermal conductivity through filling In in Ba-filled skutterudite (Ba0.3Co4Sb12). Two large ZT values of 1.33 and 1.34 have been achieved for Ba0.15In0.16Co4Sb11.83 and Ba0.14In0.23Co4Sb11.84 at 850 K, respectively. The excellent thermoelectric transport properties for BarInsCo4Sb12 are supposed to be due to the orbital hybridizations induced by In filler. It was found that the In filler made the [Sb-4](4-) rings become bigger and squarer because of an electron transition from Sb to In brought out by the orbital hybridization between In and Sb and that the Ba filler caused the [Sb-4](4-) rings to get smaller and squarer because of a reverse electron transition from Ba to Sb induced by a large difference in electronegativities between Ba and Sb. A model of how to form the rectangular [Sb-4](4-) ring is presented, and the five chemical states of Sb in CoSb3 are reasonably assigned to different chemical bonds in the model.