Nitrogen dioxide (NO2) has been attracting a lot of attention because of its toxicity and potential in contaminating land, air, and water. However, research on NO2 release behavior under oxy-fuel conditions is still insufficient. This paper aims to explore the formation and reduction mechanisms of NO2 during oxy-fuel combustion of char by using both experimental and density functional theory (DFT) methods. Results showed that with the increase of temperature, the energy barrier of the NO oxidation reaction increases, while that of the NO2 reduction reaction decreases. These facts lead to a higher peak concentration of NO and lower NO2 emissions. Because of a larger effect of temperature on the rate constant of the NO2 reduction reaction than that of the NO oxidation reaction, the NO2 concentration decreases dramatically and rapidly even if the temperature increases only slightly. As the temperature increases to above 1273 K, the equilibrium constant of the NO oxidation reaction falls to below 10(5). That is, the oxidation of NO to NO2 is incomplete and reversible over the temperature range of 1273-1673 K. By contrast, the reduction of NO2 to NO is complete and irreversible over this temperature range. As a result, almost no NO2 is observed at high temperatures. In the presence of H2O, DFT calculations show that the water-gas-shift reaction is less important in contrast to the H2O-carbon reaction, which is consistent with previous research. In this case, NOx emissions under an O-2/CO2/H2O atmosphere are lower than those under an O-2/CO2 atmosphere. Overall, combined experiments with DFT calculations offer a new approach to study the microcosmic mechanisms of NO2 formation and reduction under oxy-fuel conditions during isothermal combustion of char.