Abstract: The influences of the concentrations of nitric acid, aluminum nitrate and uranyl on the extraction of Np(Ⅳ) by 5 %TBP/kerosene at 25 ℃ are investigated, and the empirical formula is obtained at corresponding conditions. The tetravalent neptunium is extracted by means of Np(NO 3) 4 ·2TBP, and the equilibrium constant of the extraction reaction at 25 ℃ is obtained as 2.60 mol -6 ·L 6. The influence of temperature on the extraction of tetravalent neptunium is also investigated. It indicates that the extraction reaction of tetravalent neptunium is an endothermic reaction, and the free energy at 25 ℃ as well as the variety enthalpy is found to be -2.37 kJ/mol and 1.18 kJ/mol, respectively.