Abstract: Decolorization of azo dye Orange G (OG) in aqueous solution by homogeneous Fenton reaction was conducted. The effects of main parameters, including the initial concentration of OG, the dosages of ferrous ion and hydrogen peroxide (H2O2) along with the reaction temperature and the initial solution pH value, on the decolorization of the OG dye were especially investigated. The results showed that the homogenous Fenton process responsible for decolorizing OG in solution were effective and obeyed to second order kinetics with a good linear relationship (R20.989). With increasing the reaction temperature and appropriately decreasing the initial solution pH, the OG decolorization rate was enhanced accordingly. If the initial concentration of OG was appropriately decreased, the initial concentration of ferrous ion can be decreased also in ensuring to keep the decolorization efficiencies of OG to meet the environmental standard for dyes such as OG discharge concerning about the color and iron ion discharge limits.