The effect of catalyst atomic ratio on the performance of selective oxidation of p-xylene as a probe reaction over Mo-Fe catalysts prepared by sol-gel method was investigated.The catalyst evaluation results show that the optimum catalyst atomic ratio is n(Mo):n(Fe):n(Co)=2.4:1:0.02 with a high catalytic activity.The composition of the catalyst is Fe2(MoO4)3 and a little MoO3 having a cooperative effects during the oxidation of p-xylene,which was detected by XRD and FTIR.This study could provide experimental data for the technical reform of industry equipment.