Abstract: Photo-oxidation of the dye Orange Ⅱ was studied in the Fe(Ⅲ)-fumarate system under UV irradiation. The effects of pH value and initial concentrations of Fe(Ⅲ), fumarate and Orange Ⅱ were also investigated. The results showed that, lower solution pH in the range of 3.0-7.0 favors the photooxidation in the Fe(Ⅲ)-fumarate system and the removal rate of Orange Ⅱ increased with increasing initial concentration of Fe(Ⅲ). After 50 minutes, the decolorization efficiency of Orange Ⅱ and TOC solutions reached 100% and 60%, respectively, with pH was 3, c(Fe(Ⅲ)) was 25 μmol/L, c(fumarate) was 250 μmol/L and ρ(Orange Ⅱ) was 25 mg/L. The effect of initial concentrations of fumarate was not obvious on the decolorization rate of Orange Ⅱ. Under laboratory conditions, the photo-oxidation of Orange Ⅱ followed the pseudo first order reaction. Fluorescence method was taken to confirmed the production of hydroxyl radicals in the Fe(Ⅲ)-fumarate system under UV radiation, and it was helpful for the study of Orange Ⅱ decolorization mechanism.