This paper reported a numerical study on the NOx emission characteristics of opposed-jet syngas diffusion flames. A narrowband radiation model was coupled to the OPPDIF program, which used detailed chemical kinetics and thermal and transport properties to enable the study of 1-D counterflow syngas diffusion flames with flame radiation. The effects of syngas composition, pressure and dilution gases on the NOx emission of H-2/CO synthetic mixture flames were examined. The analyses of detailed flame structures, chemical kinetics, and nitrogen reaction pathways indicate NOx are formed through Zeldovich (or thermal), NNH and N2O routes both in the hydrogen-lean and hydrogen-rich syngas flames at normal pressure. Zeldovich route is the main NO formation route. Therefore, the hydrogen-rich syngas flames produce more NO due to higher flame temperatures compared to that for hydrogen-lean syngas flames. Although NNH and N2O routes also are the primary NO formation paths, a large amount of N-2 will be reformed from NNH and N2O species. For hydrogen-rich syngas flames, the NO formation from NNH and N2O routes are lesser, where NO can be dissipated through the reactions of NH + NO -> N-2 + OH and NH + NO -> N2O + H more actively. At a rather low pressure (0.01 atm), NNH-intermediate route is the only formation path of NO. Increasing pressure then enhances NO formation reactions, especially through Zeldovich mechanisms. However, at higher pressures (5-10 atm), NO is then converted back to N-2 through reversed N2O route for hydrogen-lean syngas flames, and through NNH as well for hydrogen-rich syngas flames. In addition, the dilution effects from CO2, H2O, and N-2 on NO emissions for H-2/CO syngas flames were studied. The hydrogen-lean syngas flames with H2O dilution have the lowest NO production rate among them, due to a reduced reaction rate of NNH + O -> NH + NO. But for hydrogen-rich syngas flames with CO2 dilution, the flame temperatures decrease significantly, which leads to a reduction of NO formation from Zeldovich route. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.