The full catalytic cycle that involves the oxidation of two CO molecules is investigated here by using periodic density functional calculations. To simulate the nature of Fe(OH)(x)/Pt nanoparticles, three possible structural models, i.e., Fe(OH)(x)/Pt(111), Fe(OH)(x)/Pt(332) and Fe(OH)(x)/Pt(322), are built. We demonstrate that Fe(iii)-OH-Pt stepped sites readily react with CO adsorbed nearby to directly yield CO(2) and simultaneously produce coordinatively unsaturated iron sites for O(2) activation. By contrast, the created interfacial vacancy on Fe(OH)(x)/Pt(111) prefers to adsorb CO rather than O(2), thus inhabiting the catalytic cycles of CO oxidation. We suggest that such structure sensitivity can be understood in terms of the bond strengths of Fe(iii)-OH.