Abstract: The application of aceto hydroxamic acid as complexing agent of Np(Ⅳ) for the improvement of U Np separation is studied. The effect of varying complexing agent concentration and nitric acid concentration, HDBP concentration, phase ratio, stripping times, temperature and contacting time on D (Np(Ⅳ)) in stripping process from TBP for Np(Ⅳ) are studied in detail. The partition behavior of Np in various valent states has been investigated with aceto hydroxamic acid as well. The results show that aceto hydroxamic acid is a very strong complexing agent for Np(Ⅳ).