Disclosed is a process for producing acrylonitrile from propylene, or methacrylonitrile from isobutylene by ammoxidation using an improved catalyst composition comprising an oxide catalyst represented by the formula: MomBibFefNinQqAaEeOx, wherein Q is a mixture of chromium and indium, A is at least one element selected from potassium, rubidium and cesium, E is at least one element selected from manganese magnesium, zinc, cerium, sodium and phosphorus, and m is a number of from 10 to 14, b is a number of from 0.1 to 3, f is a number of from 0.1 to 3, n is a number of from 4 to 10, q is a number of from 0.1 to 2, a is a number of from 0.01 to 0.5, e is a number of from 0 to 3, and x is a number determined by the valence requirements of the other elements present, 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. With the oxide catalyst composition of the present invention, the desired acrylonitrile or methacrylonitrile can be produced in high yield.