Ir-Ru/Al2O3 developed has shown an outstanding activity for reducing NOx by CO at low temperature in excess oxygen. However, it has remained elusive how the Ir-Ru catalyst derives this unique reactivity. Here, we tried to unveil the origin of the synergistic effect in the Ir-Ru catalyst on the lean deNOx by CO. Ir-Ru//Al2O3 showed superb NOx reduction activity along with high N2 selectivity compared to monometallic catalysts. Results of EXAFS and XRD studies indicated the formation of Ir-Ru alloy phases on the alumina. These alloy phases were likely to accelerate reaction steps required for the NOx reduction by CO, based on temperature programmed experiments together with DFT calculations. Ir-Ru//Al2O3 appeared to effectively dissociate NO on the bimetallic site, which could successively be reutilized by the facile CO-O reaction, believed to be the primary causes for the excellent deNOx activity of Ir-Ru//Al2O3 by CO under lean exhaust condition.