Quantum chemistry theoretical calculations were carried out to investigate the influence mechanism of different oxygen concentrations on the heterogeneous reduction of NO by char. The results show that there are two reaction paths for the heterogeneous reduction of NO by char using the char edge model with one hydroxyl group (R1). One is to reduce the NO adsorbed on the surface of R1 to N2O, which can be decomposed on the surface of char or react with CO. Three stepwise reactions with the highest energy barrier of 274.2 kJ/mol are found responsible for the desorption of CO and production of new active sites, on which N2O desorption can occur barrierlessly. The other path with a moderate energy barrier (139.6 kJ/mol) takes place to reduce NO to N-2. The rate-determining step in the heterogeneous reduction of NO by the char edge with two hydroxyl groups (R2), a ring opening reaction, with a higher-barrier (398.03 kJ/mol), is less effective to the reduction of NO. From the kinetic analysis it is known that the reaction rate constants increase with the increase of temperature, indicating that temperature has a great influence on the reduction of NO and a higher combustion temperature contributes to lower NO emission. At a high temperature (1800 K), the rate constant of rate-determining step of R1 is 10(3) times higher than that of R2, which shows that R1 is more beneficial to the reduction of NO. The results reveal that the mechanism of NO reduction by char is promoted by oxygen at the char surface.