Abstract: The reduction of Pu(Ⅳ) with hydroxyurea(HU) in the mixed phase of 30%TBP/OK-HNO_3 system was studied. The study results show that HU can reduce (Pu(Ⅳ)) to Pu(Ⅲ) and the reduction rate can be expressed as -dc(Pu(Ⅳ))/dt=(kc(HU))c~(-3.2)(HNO_3)c~2_(mix)(Pu(Ⅳ))c~(-1)_(mix)(Pu(Ⅲ)), where k is the rate constant, and k=(896±59) mol~(2.3)·L~(-2.3)·min~(-1) at 15 ℃. With HU serves as a reductant, 16 stages count-current cascade experiment was performed using centrifugal tube to simulate U/Pu separation in the 1B contactor of Purex process. The separation factor of Pu from U and the separation factor of U from Pu reach to 5.4×10~4 and 1.8×10~5, respectively. The amount of Pu is about 11 μg in per kg U product.