Abstract: The adsorption of NO, O₂ molecules on the surface of activated carbon fibers (ACFs) were comprehensively investigated using a simplified zigzag model based on density functional theory. The ACFs model, products, stable intermediates were optimized at B3LYP/6-31G(d) level and the catalytic oxidation mechanism of NO over ACFs surface was proposed. The results revealed that O₂ molecules are adsorbed on the ACFs surface, forming two simiquinones (C-O), and then the adsorbed NO(C-NO) is oxidized to -NO₂ by the simiquinones. On the other hand, the active oxygen atom (O^*), derived from free O₂ through catalytic process on the carbon surface, reacts with the adsorbed NO, forming -NO₂. However, O₂ adsorption is more thermodynamically favorable than NO. Based on statistical thermodynamic analysis, the transformation of NO to NO₂ was mainly attributed to the reaction between C-NO and O^* generated from the free O₂ in atmosphere.