This theoretical study investigated the oxidation reaction mechanism of acetonitrile as a volatile
organic compound by nitrogen trioxide in formation of secondary organic aerosol. The B3LYP level with 6-311++G (3df, 3pd) basis set was used to optimize geometries of reactants, products, and transition states. One product was obtained and the reaction pathway of the product was discussed and potential energy surface was plotted. The rate constant calculated at atmospheric pressure over the temperature range 300-2500 K. Finally formation of secondary organic aerosol has been discussed.