Electrical and optical properties of transparent conducting oxides (TCOs) are of essential importance for optoelectronics. Electronic structures are keys to the understanding of these properties. The geometrical and electronic structures of body-centered cubic In 2 O 3 n-typedoped by Group 14 and fifth-period main group elements (Sb, Te and I) are systematically investigated. The calculated electronic structures reveal a good hybridization between the O-2p states and the s-states of Si, Ge and Sn, resulting in superior electronic properties, such as a freeelectron-like band feature, a large band width (> 2 eV), a low effective mass (m*=0.2m 0 ) and a high electron group velocity (≥8.35×10 5 m/s). The charge localization on the dopants leads to inferior electronic properties of In 2 O 3 doped by other dopants. The calculated defect formation energy indicates that the formation of both neutral and +1 charge state Sn is spontaneous in indium oxide.