In-related nitride semiconductors exhibit characteristic behavior which can be ascribed to the large difference in the covalent radius between In and N atoms. We consider atomic and electronic structures of N mono-vacancies (V-N) in InGaN in detail by the first principles calculations. We find that the second nearest neighbor In-In interactions, which are not important in conventional semiconductors such as Si and InAs, are as crucial as the nearest neighbor In-N interaction in In-related nitride semiconductors. Moreover, we clearly show that the strong second nearest neighbor In-in interactions in InN are the physical origin of the unusually narrow band gap of InN. (C) 2009 Elsevier B.V. All rights reserved.