The mechanism of the reaction between char and nitric oxide (NO) on zigzag and armchair edge structures was studied based on density functional theory (DFT). Four possible pathways were presented to illustrate the chemical processes. The results show that the activation step and the release of carbon monoxide (CO) and carbon dioxide (CO2) are the controlling steps in the NO–char reaction. The exothermic chemisorption can release a great deal of energy which can be utilized by the subsequent reactions. The armchair edge is easier to activate (via dehydrogenation) to release CO or CO2 than the zigzag edge, while chemisorption of NO on a zigzag edge releases much more energy than that on an armchair edge. Carbon–oxygen single bond groups (C–O single bond groups) can promote the NO–char reaction by lowering the barrier of activation or producing new active sites via releasing CO2. These results are consistent with the experimental observation that C–O single bond groups can favor the NO–char reaction.