Abstract: The oxidation kinetics of depleted uranium was studied in pure oxygen pressure of 5 kPa at below 100 ℃ by using spectroscopic ellipsometry(SE). The relationship of the thickness of uranium oxide with time is measured. The results indicate that below 100 ℃ in the initial period, the relationship between oxidation layer thickness and time is parabolic. The oxidation mechanism of depleted uranium is discussed and the active energy of depleted uranium at given experimental conditions is 87.165 kJ/mol from calculation.