NOx reduction over Ir-based catalysts in the presence of excess oxygen with hydrocarbon as a reductant was investigated in the focus on observing microstructure of an Ir particle supported on various carriers and structural evolution of highly active Ir-based catalysts in the NO, reduction. Characterization of Ir-based catalysts using SEM, TEM, XRD, CO chemisorption and XPS, and reaction studies on various Ir-based catalysts have proved that the formation of a relatively large Ir metal particle with 40-60 nm of nanocrystal size carried on inert supports has been a prerequisite for the evolution of high activities in the NOx reduction rather than the formation of Lr metal state on the catalyst. Furthermore, in inert gas conditions in a high temperature range of 850-950 degrees C, Ir metal was easily formed by using the support such as TiO2 and ZrO2 that drastically decreased its specific sui;face area in the temperature range.