We have carried out the first-principle electronic structure calculations based on density functional theory on InTaO4 in various states, such as pristine, oxygen vacancy, and Ni-doped states. We have found that oxygen vacancy can induce the gap states and Ni-doping can narrow the band gap by generating additional states on the top of the valence band as well as on the top of the gap states. From our calculated results, we have shown that oxygen vacancy can do a crucial role to generate gap states which let InTaO4 absorb the visible light. (C) 2004 Elsevier B.V. All rights reserved.