This research investigates the efficiency of UV in combination with chlorine dioxide (UV/ClO2) for decolorizing of aqueous solution textile dyes methylene Blue. All the experiments have been done by using photoreactor provided with ultraviolet source of low pressure mercury-lamp (LPML). The study included the investigating the effects of dye concentration, temperature , rate flow of gas and light intensity . A complete degradation has been achieved in a somewhat short period alternatively 60 min. The maximum removal of color was found at higher temperature(318 K) . The results showed that the rate of removal increased with of decrease of initial dye concentration , and also the ratio of removal increased with increase in temperature. The activation energy for degradation of aqueous solution of methylene blue was found 16.577kJ/Mole. Moreover, it was noticed that with the increase of both air flow velocity and incident light intensity ,increases the ratio of methylene blue removal. The decolourizaition reaction was found to follow pseudo first order kinetics with respect to the dye concentration. All of the important previous effects have been applied on chemical oxygen demand (COD). The ratio of COD removal associates positively with the ratio of decolorization rate of methylene blue.