Abstract: Crystal uranyl formate is irradiated in liquid nitrogen with the selectedfrequency laser beam of infra-red 10.6μm TEA CO_2 laser. The effects of laserflux, frequency and irradiation time on the yield is studied. The difference inkinetics between the light reaction induced by the resonant absorption and thethermoreaction induced by the nonresonant absorption are made clearly. Byanalysing products of photoreaction, the composition of them is measured as UO_2and U_3O_8 with the ratio about 1:4. After dissolving U_3O_8 in HCl, U(Ⅳ) andU(Ⅵ) can be separated with ion-exchange process. Isotope enrichment of ~(235)U inU(Ⅳ)in eleven separated samples is determined, the isotope enrich factor (ε) isabout 0.0089±0.004, with the range from 0.004 to 0.017.