Vapor-phase ammoxidation of 3-picoline over 5% Cr₂O₃-Al₂O₃ catalyst was carried out at 400° and the mechanism of its reaction was studied. The reaction rate of the ammoxidation of 3-picoline depended on the partial pressure of 3-picoline and oxygen, but was not affected by that of ammonia. The reaction therefore is controlled by oxidation process. Nevertheless the kinetic behavior was different from the oxidation of 3-picoline over the same catalyst. In order to clarify this phenomenon, the ammoxidation and oxidation of 3-picoline over γ-Al₂O₃, 2% or 5% K₂O-5% Cr₂O₂-Al₂O₂, and 10% Cr₂O₃-Al₂O₃ were examined, and the oxidation of pyridine with and without ammonia was studied. From these experiments the following results were obtained. In the absence of ammonia, the total oxidation proceeds, because chromium is in the higher oxidation state and the carrier, γ-Al₂O₃, takes part in the reaction. In contrast, in the presence of ammonia, Cr₂O₃-Al₂O₃ becomes moderately active so that the total oxidation does not proceed and 3-cyanopyridine is obtained selectively.