Abstract: To explore the application of U (Ⅳ) as a reductant in spent fuel reprocessing, investigations are carried out on photochemical reduction of U(Ⅵ) in the system of UO2 (NO3)2-HNO3-N2H5NO3 in which hydrazine serves not only as a photoreductant, but also as a stabilizer for U (Ⅳ). An argon ion laser with monochromic wave length of 4880A is used as the light source. The effect of various factors on the photochemical reaction, such as power of light, temperature and composition of solution is investigated. The initial rate of the formation of U (Ⅳ) and the quantum yield of U (Ⅳ) are calculated from experimental data. The analyses of the photochemical products show that U(Ⅴ) is formed in the course of reaction, and U(Ⅳ), N2 and NHf are obtained at the end of the photochemical reaction. The mole ratio of final products U (Ⅳ), N2 and NH4+ is 1:1:2. A possible mechanism of the photochemical reaction is suggested.