The dilution effect of air stream according to agent type on flame structure and NO emission behaviour is numerically simulated with detailed chemistry in CH4/air counterflow diffusion flame. The volume percentage of diluents (H2O, CO2, and N-2) in air stream is systematically changed from 0 to 10. The radiative heat loss term, based on an optically thin model, is included to clearly describe the flame structure and NO emission behaviour especially at low strain rates. The effect of dilution of air stream on the decrease of maximum flame temperature varies as CO2 > H2O > N-2, even if heat capacity of H2O is the highest. It is also found that the addition of CO2 shows the tendency towards the reduction of flame temperature in both the thermal and chemical sides, while the addition of H2O enhances the reaction chemically and restrains it thermally due to a super-equilibrium effect of the chain carrier radicals caused by the breakdown of H2O in high-temperature region. The comparison of the nitrogen chemical reaction pathway between the cases of the addition of CO2 and H2O clearly displays that the addition of CO2 is much more effective to reduce NO emission. Copyright (C) 2002 John Wiley Sons, Ltd.