The oxidation behavior of the simulated spent fuel of burn up 33 MWD/kgU was investigated to predict that of the spent fuel in the temperature ranges of 400 to $700^{circ}C$ and was compared with those of $UO_2$. The forms of uranium oxides after the oxidation were conformed by XRD analyses. The oxidation rate at each the temperature and the activation energy were obtained. After complete oxidation, the simulated spent fuel was converted to $U_3O_8$ and pulverized to powder due to the density difference between the simulated spent fuel and uranium oxides. The activation energies were 85.35 and 30.77kJ/mol in the temperature ranges of 400$leq$T($^{circ}C$)$leq$500 and 500$leq$T($^{circ}C$)$leq$700, respectively.