Abstract: Density functional theory calculations have been used to study the adsorption of Iridium on MgO(100) surfaces. The electronic structures of the bare and the Ir adsorbed MgO(100) surfaces have been analyzed. By modeling different adsorption sites, one can conclude that the energetically most favorable site for the Ir atom is the top site of the oxygen in the MgO(100) surface. Density of states and induced charge density illustrate that different from those for an isolated Ir atom in vacuum, the 5d states of the adsorbed Ir atoms are active. By interacting with the surface oxygen, the 5d states of the adsorbed Ir atom are split. The energy levels of these states are close to the Fermi level, strong p-d hybridization can be observed between the Ir-5d orbital and the O-2p orbital, which play an important role in the catalytic performance of the system with Ir atoms adsorbed.