Abstract: Disclosed is an ammoxidation catalyst composition for use in producing acrylonitrile from propylene, or methacrylonitrile from isobutylene, by ammoxidation of the propylene or of the isobutylene, comprising an oxide catalyst and a silica carrier having the oxide catalyst supported thereon, wherein the silica carrier is present in an amount of from 40 to 60% by weight, based on the total weight of the oxide catalyst and the silica carrier. The oxide catalyst comprises oxides of molybdenum, bismuth, iron, and component A which is at least one element selected from potassium, rubidium and cesium, wherein bismuth, iron and component A are, respectively, present in amounts of from 0.1 to 6, from 0.1 to 8 and from 0.01 to 0.5 in terms of atomic ratios relative to twelve atoms of molybdenum.