The hydrogen peroxide role in photocatalytic degradation of an anionic azo dye, Acid Orange 7 (AO7), was investigated in a slurry reactor. Commercial ZnO nanoparticles with an average size between 10 to 30 nm were used as catalysts. Optimum conditions for different parameters, including dye concentration (10-100 mg/L), catalyst concentration (0.1-0.5 g/L), and pH (5-10), were determined first in the absence of H2O2. Changes in the COD were measured for the optimum condition. The impact of adding hydrogen peroxide at different concentrations to the system operating at optimum conditions was investigated. It was observed that 0.416 mM hydrogen peroxide increased the system's efficiency and decreased reaction time by 40 min. The reaction followed first-order kinetic. Hydrogen peroxide alone did not contribute to oxidizing the contaminant, and its positive impact was attributed to decreasing electron-hole recombination in the photocatalytic process. Not only can the hydrogen peroxide-assisted photocatalytic process decrease retention time in treatment units, but it can also result in more contaminant degradation. Therefore, it can reduce the treatment cost.