Abstract: A process for producing a fluid catalyst having good activity from a coarse catalyst wherein said fluid catalyst is suitable for oxidation, ammoxidation or oxidative dehydrogenation reaction of hydrocarbons which comprises crushing a fluid catalyst, a greater part of which is composed of particles having a large particle size, to produce finely-divided particles, adjusting the distribution of particle size of said catalyst so that the amount of particles of a size of about 20.mu. or less is about 30% by weight or less of the total weight of catalyst particles and the amount of particles having a weight average particle size is 90% or less based on the catalyst prior to crushing, and thereafter calcining said catalyst at a temperature of about 500.degree. C. - about 1000.degree. C. under a non-reducing atmosphere.