The purpose of this study was to investigate the NOx formation and reduction mechanisms in staged O-2/CO2 combustion and in air combustion. A flat CH4 flame doped with NH3 for fuel-N was formed over the honeycomb, and NOx formation characteristics were investigated. In addition, chemiluminescence of OH* distribution was measured, and CHEMKIN-PRO was used to investigate the detailed NOx reduction mechanism. In general, the NOx conversion ratio decreases with decreasing primary O-2/CH4 ratio, whereas NH3 and HCN, which are easily converted to NOx in the presence of O-2, increases rapidly. Therefore, a suitable primary O-2/CH4 ratio exists in the staged combustion. Our experiments showed the primary O-2/CH4 ratio, which gave the minimum fixed nitrogen compounds in O-2/CO2 combustion, was lower than in air combustion. The NOx conversion ratio in O-2/CO2 combustion was lower than in air combustion by 40% in suitable staged combustion. This could be explained by high CO2 concentrations in the O-2/CO2 combustion. It was shown that abundant OH radicals were formed in O-2/CO2 combustion through the CO2 + H -> CO + OH, experimentally and numerically. OH radicals produced H and O radicals through H-2 + OH -> H + H2O and O-2 + H -> OH + O, because a mass of hydrogen source exists in the CH4 flame. O and OH radicals formed in the fuel-rich region enhanced the oxidation of NH3 and HCN. NOx formed by the oxidation of NH3 and HCN was converted to N-2 because the oxidation occurred in the fuel-rich region where the NOx reduction effect was high. In fact, the oxidation of NH3 and HCN in the fuel-rich region was preferable to remaining NH3 and HCN before secondary O-2 injection in the staged combustion. A significant reduction in NOx emission could be achieved by staged combustion in O-2/CO2 combustion. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.