Abstract: Effects of concentraction of uranium(Ⅳ), nitric acid, uranium(Ⅵ)and different diluents on the formation of the third phase are investigated in extraction of uranium(Ⅳ)and nitric acid by 30%TBP.The equilibrium concentrations of uranium(Ⅳ)and nitric acid in aqueous phase, heavy organic phase and light organic phase are determined respectively. The equilibrium concentrations of TBP in heavy organic phase and light organic phase are also determined. The experimental results show that if uranium(Ⅵ) exists in aqueous system of U(NO 3) 4 HNO 3,the extraction distribution of U(Ⅳ) and HNO 3 in organic phase decreases and the third phase disappears at last, when the concentration of uranium(Ⅵ) in U(NO 3) 4 HNO 3 solution is high enough. When benzene, derivate of benzene, chloroform, carbon tetrachloride and lower linear chain saturated hydrocarbon are used as diluent, the third phase is not easily formed. However, as hiigher linear chain saturated hydrocarbon is used as diluent, the third phase is easily formed.