Abstract: Several neutral organophosphorous extractants with different substituents are synthesized with Michaelis' method. They are as follows:(1) (n-C8H17O)2(C2H5) PO: (2) CH3(CH2),CH(CH3)O2(C2H5) PO; (3)CH3(CH2)4CH(C2H5)CH2O2(C2H5)PO;(4)CH3(CH2)4CH(C2H5)CH2O2 (CH3).PO;(5)CH3(CH2)4CH(C2H5)CH2O2(CH3)2CHPO;(6)CH3(CH2)4 CH(C2H5)CH2O2CH3)2CHCH2PO.The purity and structure of these extractants are determined with the methods of IR, NMR and mass spectra. The extraction of U(Ⅵ) and Pu(Ⅳ)from nitric acid solution by these extractants is studied as a function oftemperature in the rang of 0-50℃,and the complex formabilityof U(Ⅵ)and Pu(Ⅳ) in nitric acid medium are determined at different temperatures. From the variation of the equilibrium constants with temperature the thermodynamic quantities AG, AH and AS are calculated for these extractants. The results indicate that the value of the enthalpy change is negative for the extraction of both metals and the value of entropy change is negative for the extraction of U(Ⅵ) and positive for the extraction of Pu(Ⅳ) . The steric effect of the substituents plays a major role in the difference of the extractionability of different extractants. Those with sterically effective bulky groupscan improve the separation of U(Ⅵ) and Pu(Ⅳ) to some extent.