This paper used the opposed-flow flame model and GRI 3.0 mechanism to investigate NO emission characteristics of H-2-rich and H-2-lean syngas under diffusion and premixed conditions, respectively, and analyzed influences of adding H2O, CO2 and N-2 on NO formation from the standpoint of thermodynamics and reaction kinetics. For diffusion flames, thermal route is the dominant pathway to produce NO, and adding N-2, H2O and CO2 shows a decreasing manner in lowering NO emission. The phenomenon above is more obvious for H-2-rich syngas because it has higher flame temperature. For premixed flames, adding CO2 causes higher NO concentration than adding H2O, because adding CO2 produces more O radical, which promotes formation of NO through NNH + O = NH + NO, NH + O = NO + H and reversed N + NO = N-2 + O. And in burnout gas, thermal route is the dominant way for NO formation. Under this paper's conditions, adding N-2 increases the formation source of NO as well as decreases the flame temperature, and it reduces the NO formation as a whole. In addition, for H2-lean syngas and H-2-rich syngas with CO2 as the diluent, N + CO2 = NO + CO plays as an important role in thermal route of NO formation. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.