Abstract: Based on the BET theory and the absorption behavior with a single molecular-form between metal oxide and N_2 at a favorable temperature, the specific area of U_3O_8 powder is determined. The variation of pressure due to the absorption was measured, and the slops and intercepts of the specification curve fitting are obtained by standardizing quantitative gas ball. Then the measured sample's area with the single molecular-form is calculated according to the pressure variation and fitting parameters. The precision of this method can be better than 11%.