Abstract: In this paper, the spectrophotometric determination of micro amount of uranium from the large amount of thorium with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) is investigated.Uranium is extracted from the large amount of thorium in 1 mol/1 HNO3 solution by di-(l-methylheptyl) methyl phosphate (DMHMP)in benzene. Then, the extracted uranium in organic phase (DMHMP) is determined directly by the spectrophotometric method with 5-Br-PADAP in ethanol medium. The apparent molar absorption of uranium at 575 nm is 7.0×104 1·mol-1·cm-1. Beer's law is obeyed within 10-60 μg of uranium in 25 ml solution.The precision of this method is within 1.0%. The recovery of uranium is over 98% (U=10-60μg) . The method is applied to the study in reprocessing of Th-U fuel. When concentration ratio of Th to U is lower than 2000, thorium doesn't influence the determination of uranium.