Due to the accident of Fukushima Daiichi nuclear power plant, fuel debris has been produced inside the nuclear reactor vessels. To reprocess fuel debris, a pyroprocess of selective fluorination and a molten salt electrolysis have been proposed. The aim of this study is to investigate the fluorination behavior of uranium - zirconium oxide mixture. The samples have been prepared as follows. First, UO2 was made by the reduction using Ar - H2 mixture gas for U3O8 at 1000°C. Second, UO2 and ZrO2 were mixed by the molar ratios of some. Finally, these samples were treated in the environment of either oxidation or reduction. To evaluate fluorination behavior of these mixtures, a thermogravimetric differential thermal analyzer and a powder X-ray diffractometer. Depending on pre-treated environment of the mixtures, the difference of fluorination behavior is identified. How much degree of fluorination is the best to introduce into molten salt is going to under consideration.