Abstract: The deactivation mechanism of Fe-Sb-W oxides catalyst for ammoxidation of pro-pylene has been studied by ESR.The amount of coke deposited on the surface of cata-lyst and the changes of structure of the catalyst were determined simultaneously. It was shown that the reaction FeSbO₄+e→α-Fe₂O₃+Sb₂O₃ took place when the amount of deposited coke exceeded a certain limit and the active structure in the surface of the catalyst was deteriorated.The catalyst was then deactivated seriously,the conversion of propylene and yield of acrylonitrile were only 40% and 9% respectively, while the main product was CO₂.