Abstract: In this paper a spectrophotometry using arsenazo III is used to determine traceTh in U_3O_8. The sample of U_3O_8, is dissolved in HNO_3 and then turned into HClmedium. Uranium is absorbed in TBP-levextrel at 1:1 HC1 medium and thoriumis eluted by 1:1 HCl. The absorbtion obeies the Beer law when the content of Th is in the range of0.125--1.25 μg/10 ml. If the uranium quantity of the U_3O_8 sample is 250 mg andthe content of Th is 1 ppm., the precision is ±6% and the recovery is 98%. Thedetected limit is 0.5 ppm six samples can be performed within eight hours for oneoperator.