K ion-added MgO catalysts were prepared by using KOH, KNO3 or KCl as the precursor of K ion and applied to the coupling reaction of methanol and acetonitrile to form acrylonitrile. The reaction proceeds via acetonitrile carbanion intermediate which is formed on surface base site. The catalytic activity calculated by acetonitrile conversion was in the order KOH-added MgO > KNO3-added MgO > KCl-added MgO which was in agreement with the trend of basic strength of the catalysts measured by CO2-TPD. However, the selectivity to acrylonitrile was in the opposite order and the main by-product was propionitirle, the hydrogenated form of acrylonitrile. This suggests that hydrogenation also occurred on base site in this reaction. When CO2 was introduced in the reaction system, acetonitrile conversion was increased suggesting that CO2 played a role of mild oxidant in this reaction. Acetonitrile conversion and acrylonitrile selectivity over KNO3-added MgO were 22.3 % and 79.0%, respectively.