Abstract: The synergistic extraction of U(Ⅵ) and Th(Ⅳ) from nitric acid solution by HBMPPT(4 benzoyl 2,4 dihydro 5 methyl 2 phenyl 3H pyrazol 3 thione) and PSO(Petroleum Sulfoxide)in toluene is studied. The extraction ability of HBMPPT is not as high as that of its parent (HBMPP),but the addition of a little amount of PSO significantly improved the extraction ability to U(Ⅵ),while the ability to extract Th(Ⅳ) is sliqhtly increased. The synergistically extracted complexes may be presented as UO 2(BMPPT) 2·PSO for U(Ⅵ),and Th(NO 3) 2(BMPPT) 2·PSO for Th(Ⅳ), respectively. The separation coefficient is about 1000 under certain condition when HBMPPT PSO is used to separate U(Ⅵ) and Th(Ⅳ).