Abstract: The stability of U(IV) nitrate in separated organic solution (30% TBP) and system of two phases (organic and aqueous)was studied. The results show that the oxidation of U(IV) in TBP phase is completed in about 5-7 hours. The concentration of HNO2 in the TBP phase gradually increases during the oxidation of U(IV). The losses of U(IV) are about 10-20% during cascade experiment containing uranium and nitric acid.The effects of some factors such as concentration of Pu(IV) and U(IV) in the TBP phase,concentration of HNO3 and N2H4 in aqueous phase,and U(IV)/Pu(molar ratio) on back-extraction of Pu(IV) were investigated.Through cascade experiment with simulated 1BF feed, the effects of some factors such as U(IV)/Pu (molar ratio) ,feeding point of U(IV) feed,the acidity of aqueous phase,the o/a phase ratios in the reduction zone on the separation of U/Pu were investigated. The results show that when U(IV)/Pu (molar ratio) is 8,5,3 and 2,the DF for removal of Pu from U product is always greater than or equal to 1. 36×104.