The NO formation characteristics and reaction pathways of opposed-jet H-2/CO syngas diffusion flames were analyzed with a revised OPPDIF program which coupled a narrow band radiation model with detailed chemical kinetics in this work. The effects of strain rates ranging from 0.1 to 1000 s(-1) and diluents including CO2, H2O and N-2 on NO production rates were investigated for three typical syngas compositions. The numerical results demonstrated that NO is produced primary through NNH-intermediate route and thermal route at high strain rates, where the reaction of NH + O = NO + H (R51) also become more active. Near the strain rate of 10 s-1, the flame temperature is the highest and thermal route is the dominant NO formation route, but NO would be consumed by reburn route where NO is converted to NH through HNO, especially for H-2-rich syngas. At low strain rates, radiative heat loss results in a lower flame temperature and further reduce NO formation, while the reaction of N + CO2 = NO + CO (R140) become more important, especially for CO-rich syngas. With the diluents, NO production rates decreased with increasing dilution percentages. When the flame temperature is very high as the thermal route is dominant near strain rate of 10 s(-1), CO2 dilution makes flame temperature and NO production rate the lowest. Toward both lower and higher strain rates, adding H2O is more effective in reducing NO because R140 and NNH-intermediate route are suppressed the most by H2O dilution respectively. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.