Abstract: The influence of the initial concentration of Np(Ⅵ) in nitric acid solution, temperature and acidity etc. on the photoreduction of Np(Ⅵ) has been studied in detail with mercury lamp. The rate equation of photoreduction of Np(Ⅵ) is formulated as follows._dNp(Ⅵ)/dt =0.05766Np(Ⅵ)H+-1The photochemical reaction mechanism of Np suggested in this system can be interpreted satisfactorily by the experimental results.