Numerical study is conducted to understand the impact of fuel composition and flame radiation in flame structure and their oxidation process in H-2/CO synthetic gas diffusion flame with and without CO2 dilution. The models of Sun et al. and David et al., which have been well known to be best-fitted for H-2/CO synthetic mixture flames, are evaluated for H-2/CO synthetic mixture flames diluted with CO2. Effects of radiative heat loss to flame characteristics are also examined in terms of syngas mixture composition. Importantly contributing reaction steps to heat release rate are compared for the synthetic gas mixture flames of high contents of H-2 and CO, individually, with and without CO2 dilution. The modification of the oxidation pathways is also addressed. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.