The wastewater from textile dyeing industry is difficult to be treated successfully according to both high variability of composition and color intensity. To investigate the effects of reactor gap size and UV dosage on the decolorization of dye wastewater, a commercially available azo dye C.I. Acid Blue 113 was chosen as a model compound. UV/H2O2 processes with various gap sizes and setups of plug flow reactor and recirculated batch reactor were proposed to deal with the dye wastewater in this study. The experimental parameters including the design of reactor configurations of annular gap size, and in batch system or plug flow reactors and hydrogen peroxide dosage, UV dosage were investigated. The gap size of reactor was adjusted by different diameter of reactor shells in order to optimize the reactor configuration. The color removal percentage was used to evaluate the treatment efficiency. An optimal hydrogen peroxide concentration of 46.53 mM was observed in this study for highest decolorization rate. Besides, the pseudo-first-order rate constant of 3.14 min(-1) was obtained by plug flow reactor with 0.5 cm gap size, 120.70 W/l of UV dosage and 23.27 mM of H2O2 dosage. The first-order rate constant, which was about 20 times less than that of plug flow reactor, was obtained 0.1422 min(-1) by recirculated batch reactor with 2.0 cm gap size, 7.0 W/l of UV and 23.27 mM of H2O2 dosages. Ultimately, we developed an effective pre-treatment or treatment technology for dye wastewater to provide the dyeing industries and dye manufacturers an alternative to meet the effluent standards.