Numerical analysis on dame structure in a counterflow diffusion flame has been conducted for understanding the effects of CO2 addition to fuel, systematically varying initial concentration of CO2 and axial velocity gradient, The effects of CO2 addition to fuel side in a counterflow diffusion flame are globally divided into two categories: diluent effects due to the relative reduction in the concentrations of the reactive species, and direct chemical effects caused by the breakdown of CO2 through the reactions of third-body collision and thermal dissociation. The deflection of CO2 mole fraction profile with mixture fraction clarifies that the converted CO quantity from CO2 is not negligible at low axial velocity gradients. It is also known that the addition of CO2 does not alter the basic skeleton of the H-2-O-2 reaction mechanism, but contributes to the formation and destruction of hydrocarbon products such as HCO, At high axial velocity gradients the CO converted reaction is suppressed and then CO2 plays the role of a diluent at these conditions. Copyright (C) 2001 John Wiley & Sons, Ltd.