Three-dimensional direct numerical simulations (DNS) of high hydrogen content (HHC) syngas nonpremixed jet flames with a Reynolds number of Re = 6000 have been carried out to study the nitric oxide (NO) pollutant formation. The detailed chemistry employed is the GRI 3.0 updated with the influence of the NCN radical chemistry using flamelet generated manifolds (FGM). Preferential diffusion effects have been considered via FGM tabulation and the reaction progress variable transport equation. The DNS based quantitative results indicate a strong correlation between the flame temperature and NO concentration for the pure hydrogen flame, in which NO formation is mainly characterised by the Zeldovich mechanism. The results also indicate a rapid decrease of maximum NO values in H-2/CO syngas mixtures due to lower temperatures associated with the CO-dilution into H-2. Results on NO formation routes in H-2/CO syngas flames show that while the Zeldovich mechanism dominates the NO formation at low strain rates, the high NO formation rate at high strain rates is entirely caused by the NNH mechanism. We also found that the Fenimore mechanism has a least contribution on NO formation in H-2/CO syngas flames due to absence of CH radicals in the oxidation of CO. It is found that, due to preferential diffusion, NO concentration exhibits higher values near the flame base depending on the hydrogen content in H-2/CO syngas fuel mixture. Crown Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.