In this study, the secondary effluent of biologically treated dye manufacturing wastewater was treated by H2O2/UV oxidation process. The efficiencies of COD and color removal were evaluated, and this was followed by the calculation of operating cost per unit water treated. Based on these information, this process was to be evaluated for the possibility of replacing the post coagulation and sodium hyper chlorite oxidation for decolorization purpose. The wastewater was taken from a local dye manufacturing plant. The photoreaction system belongs to a recirculated flow mode using one or six UV lamps. Major process parameters such as the dosage of H202, single or multiple step addition of H2O2, and aeration were investigated for their effect on the process performance. As a result, the maximum COD or color removal of 53% and 95% can be obtained at sampling time of 60 mm, respectively, and they become 84% and near 100% at time of 120 mm. It is also indicated that the removal of color is much faster than that of COD. Under the requirement of reducing COD down to 80 mg/l, the unit operating cost including chemical and electricity costs ranges from 41 to 49 NT$/m3 for different operating conditions. In addition, it is suggested to add H2O2 in one step, instead of multiple steps, and to avoid the use of aeration in the process.