Abstract: Carbon deposition and catalyst deactivation of a Sb-Fe oxide catalyst for ammoxidation of propylene were investigated by means of a fluidized-bed reactor charged with 10 ml catalyst. The reactivity, surface acidity, specific surface area, average pore radius and ESR spectrum data were obtained with the catalysts of increasing carbon deposition. Chemisorption of oxygen and propylene and TPD (thermal programmed desorption) of propylene were carried out on the commercial Mo-Bi and Sb-Fe oxide catalysts. Isotopic experiment using acrylonitrile-2-14C as tracer was also carried out. The following results were obtained. 1 ) Carbon deposition due to intermediates and acrylonitrile on selective oxidation sites could be the primary reason for catalyst deactivation;2 ) Sb-Fe oxide catalyst has a property of easy reduction and difficult reoxidation, so that the active catalyst structure-FeSbO4 would be decomposed under oxygen-poor condition into Sb2O3 and 2-Fe2O3 which forms the deep oxidation catalyst component. A suggestion for improving catalyst properties has been proposed from these results.