Numerical study, aimed at the understanding of the flame structure in O-2/CO2 recycling combustion system, has been conducted with detailed chemistry. Special concern is focused on addition effect of carbon dioxide on flame structure in H-2-O-2 counterflow diffusion flame as a simulating configuration. To clarify chemical and thermal effects on flame structure, the comparison between predicted results with a virtual species X to displace the real carbon dioxide and with added carbon dioxide in oxidizer stream is made according to strain rate and the concentration of added CO2. From the systematical comparison of a dominant radical producing reaction with a chain termination reaction the effects of strain rate and composition control of oxidizer stream on flame structure are estimated. It is found that the behaviours of C-1- and C-2-branch species are a direct outcome of that of produced CO due to the breakdown of added CO2. There exists a temperature dependency in the behaviour of produced CO and this competes for the behaviour of the produced CO with chemical effects due to the backward reaction of CO + OH = CO2 + H. Copyright (C) 2003 John Wiley Sons, Ltd.