Abstract: The source of increase in weight of U3O8 during storage is investigated The main source comes from the absorption of water in atmosphere Probably U3O8·H2O, U3O8·2H2O and U3O8·3H2O are formed.The increase in weight is related to methods of preparation, storage period and conditions during storage It is shown that the process of water absorption is irreversible Therefore it is advisible to store the U3O8 in ampoule, or to ignite U3O8 at 850℃ in muffle furnace for two hours before use.