Aims: The aim of the present study is to investigate the efficiency of ultraviolet/ nickel oxide (UV/NiO) system as one form of advanced oxidation processes (AOP) for decolorization of red poly azo. Materials and Methods: This study was conducted as a laboratory scale in a batch mode. Ultraviolet radiation was provided by a low pressure (11 W) UV lamp. Effects of various factors including pH, different irradiation durations, different concentration of nickel oxide, and initial dye concentration were evaluated. Results: The results of the UV/NiO system's assessment showed that UV light alone cannot remove DR 80 dye. Nickel oxide is an effective catalyst in the decolorization of dye with the nanophotocatalytic process. The decolorization efficiency increases with decreasing pH value and the optimum pH value is 4. Fainally, the highest removal efficiency achieved by UV/NiO process for DR 80 dye with concentrations of 25 mg/l and 50 mg/l was 94.3% and 82.2%, respectively. UV/NiO-based decolorization process follows pseudo-second-order reaction kinetics. Conclusions: From the findings of the present study, it can be concluded that UV/NiO process is an effective technique for decolorization of poly azo dye, DR 80, in aqueous solutions.