Oxy-fuel combustion has been deemed a promising technology to reduce CO2 emissions from coal-fired power stations, and oxy-steam combustion technology is regarded to be the next-generation oxy-fuel combustion technology. The characteristics and mechanism of NO formation were numerically studied in the counter-flow laminar diffusion flame using methane as fuel. The comparisons of the N conversion rate in the oxy-steam combustion with that in air combustion under a similar temperature profile show that the emission of NO is enhanced due to high concentration of H2O during oxy-steam combustion compared with air combustion. The chaperone effect of H2O and a high mole fraction of H2O are important reasons for the enhancement of NO formation during oxy-steam combustion because H + NO + M double left right arrow HNO + M and HNO + OH double left right arrow NO + H2O are enhanced dramatically. The key elementary reaction for the enhancement of NO production during oxy-steam combustion is NH + H2O double left right arrow HNO + H-2 because it produces a large amount of HNO due to a high mole fraction of H2O. The nitrogen conversion rate through the pathway NH2 -> NH -> HNO -> NO is dramatically enhanced and predominant during oxy-steam combustion. The nitrogen conversion rate increases with the increase in the mole fraction of H2O in O-2, and equivalence ratio has a minimal impact on the NO formation during oxy-steam combustion. (C) 2015 Elsevier Ltd. All rights reserved.