Density functional theory calculations have been used to study the adsorption of CO on the Ir/MgO(100). Substrate and adsorbate geometry relaxation, adsorption, energy and density of states are computed. Top and bridge adsorption sites for iridium atom deposition have been considered. The result indicated that the energetically most favorable sites are the top sites of the O atoms. CO adsorption on the Ir atom is then studied. Different adsorbate arrangements, perpendicular and parallel to the surface have been considered. Among them, the most stable CO chemisorption geometry, is that corresponding to the adsorbate perpendicular to the surface iridium atom and C-down oriented. Density of states and the CO stretching frequency analysis further found that the obvious charge transfer from adsorbate to the substrate. (C) 2011 Elsevier B.V. All rights reserved.