Abstract: The critical concentration of uranium(Ⅳ), c (U(Ⅳ)) max , in pregnant organic phase is determined for the extraction of U(Ⅳ) by 30%TBP kerosene from U(Ⅳ) HNO 3 solution, c (U(Ⅳ)) max decreases gradually with increase of concentration of HNO 3 and reaches a minimum value at 4.0～5.0 mol/L HNO 3,then increases gradually. c (U(Ⅳ)) max increases proportionally with the temperature. c (U(Ⅳ)) max in 30%TBP diluent is determined too, c (U(Ⅳ)) max decreases with increase of carbon numbers of linear chain saturated hydrocarbon.The curve of critical concentration of U(Ⅳ) vs concentration of HNO 3 is obtained.If initial concentration of HNO 3 is 3.0 mol/L at of 20℃,critical initial concentration of U(Ⅳ) is 0.158 mol/L.However,when critical initial concentration of U(Ⅳ) is more than 0.158 mol/L in 3.0 mol/L nitric acid,the third phase will be formed for extraction of U(Ⅳ) with 30%TBP kerosene.Distribution ratio of uranium ( D (U(Ⅳ))) and nitric acid ( D (H +)) are determined,and effects of concentration of uranium(Ⅳ),nitric acid and different diluents on D (U(Ⅳ)) are discussed.