In the present study, the effects of the composition and nitrogen dilution level on the precise structure and NO. formation characteristics were numerically investigated for ternary syngas composed of H-2, CO, and CH4 in laminar counterflow non-premixed flames. Prior to the parametric study, in order to evaluate the prediction capability for the NOx formation characteristics of syngas/air counterflow non-premixed flames, the GRI 3.0 mechanism was validated against measurements. Numerical results indicate that the GRI 3.0 mechanism yields good overall agreement with the measured peak level and distribution of the NO mole fraction over a relatively wide range of H-2/CO compositions and strain rates. Computations were carried out for wide ranges of ternary syngas composition and nitrogen dilution level. Based on the numerical results, the detailed discussions are made for the essential features and precise structure of the non-diluted and nitrogen-diluted CH4/CO/H-2 syngas counterflow diffusion flames including the temperature, species mole fractions, and NO production rate and its integral. Special emphasis was given to the effects of the fuel-side nitrogen dilution on the NO formation characteristics of the CH4-rich, CO-rich, and H-2-rich syngas diffusion flames for the different ternary syngas compositions. The dominant physical processes and chemical paths to influence the precise flame structure and NO generation encountered under the actual syngas combustion conditions were systematically analyzed and identified. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.