Abstract: A method is disclosed for producing fine size molybdenum trioxide powder which involves heating ammonium dimolybdate in ambient atmosphere at a temperature of from about 455.degree. C. to about 465.degree. C. for a time of from about 5 hours to about 6 hours to produce a first molybdenum trioxide which is granulated to about -10 mesh and then heated in a furnace in which there are three heating zones. The first molybdenum trioxide is heated in the first heating zone at a temperature of from about 550.degree. C. to about 590.degree. C., in the second heating zone at a temperature of from about 580.degree. C. to about 610.degree. C. and thereafter in the third heating zone at a temperature of from about 580.degree. C. to about 625.degree. C., with the feed rate of material in the three zones being from about 1.5 to about 3.0 kg/hr to produce a yellow molybdenum trioxide. The yellow molybdenum trioxide is then cooled and granulated to about -40 mesh.