Supported Ir catalysts effectively catalyzed the NO reduction with CO and C3H6 under slight lean and high-temperature conditions, although supported Pt and Pd showed the activity for NO reduction in low temperature region with narrow window. The NO reduction activity of supported Ir catalysts was quite different depending on the support oxide. Among them, Ir/SiO2 and Ir/CeO2 were the highest and lowest active catalysts, respectively. Although no relationship between the reducibility estimated by H-2-TPR and the NO reduction activity was observed, TPO measurements revealed that the stability of Ir species in catalytically active reducing state is a key factor to determine the NO reduction activity. In situ FT-IR spectroscopy confirmed the formation of catalytically active reduced Ir species during the reaction. From the comparison of the wavenumber of IR band due to adsorbed CO species with the NO reduction activity, too strong Ir - support interaction causes the formation of stable Ir oxide species, resulting in the depression of NO reduction activity. The role of support oxide for Ir catalyst was concluded to stabilize the Ir species in catalytically active reducing state created during the reaction via the Ir - support interaction.