To apply the fluoride volatility process to the spent nuclear fuel, fluorination of UO2 by fluorine has been studied. In this reaction, it is possible that the U-O-F compounds, such as UO2F2, are produced. Therefore, study of such compounds is useful in order to know the fluorination behavior of UO2. This paper presents the fluorination behavior of UO2F2 in the presence of F2 and O2, analyzed by thermogravimetry and differential thermal analysis (TG-DTA) method using anti-corrosion type differential thermobalance. In fluorine gas, exothermic peaks appeared and volatilization of UF6. In oxygen gas, only slowly pace decomposition was measured from UO 2F2 to UF6 and UO3.