K/MgO-catalyzed coupling reaction of methanol and acetonitrile to form acrylonitrile was studied. In non-oxidative atmosphere, the doping of K enhanced the catalytic activity of MgO resulting in the increase of acetonitrile conversion. However, propionitrile, the hydrogenated form of acrylonitrile, was the main product. Thermodynamic analysis showed that oxidative condition is necessary to suppress the production of propionitrile and raise the acrylonitrile selectivity. When oxygen was introduced in the reaction system, both acetonitrile conversion and acrylonitrile selectivity were increased. Oxygen facilitated acrylonitrile synthesis reaction and prevented the hydrogenation of acrylonitrile to propionitrile. However, introduced oxygen caused overoxidation of reactants and products to CO_x. In case of using carbon dioxide as a mild oxidant, overoxidation was not appreciably observed and catalytic activity was also enhanced.