Abstract: The synergistic extraction of uranyl nitrate with binary system of 1-phenyl-3-methyl-4-benzoyl-pyrazolone-5 (PMBP) and tributyl phosphate (TBP) or diphenyl sulfoxide (DPSO), and ternary system of PMBP-DPSO-TBP were studied. The binary synergistic equilibrium constants were determined to be log β12=3.58 (PMBP-TBP system) and log β13 = 3.61 (PMBP-DPSO system)respectively. Based upon the principle of saturated coordination number, a coordinate water molecule might be present in the structural formula of UO2A2·B, and it should be replaced by a neutral extractant to decrease the hydrophilic ability. It was found that synergic effect occured during the extraction of uranium (Ⅵ) with the -benzene solution of PMBP4+DPSO+TBP. The formation of a ternary extracted complex UO2A2·DPSO·TBP was confirmed by the usual slope method. A method was proposed for the calculation of the equilibrium constant which was evaluated to be logβ/123=5.58.