This study present the removal of 4-chlorophenol (4-CP) from water by nickel oxide (NiO) nanoparticles. Photodegradation was compared using different processes in a batch reactor with medium-pressure mercury lamp irradiation. The effects of operating conditions such as feed concentration of reactants, catalyst load, pH, and half-life time on photodegradation system were evaluated. The results showed that photodegradation using UV/NiO system in the presence of H₂O₂ were much more effective than using either UV/H₂O₂ or UV/NiO processes. The optimum conditions for the complete degradation of 4-CP were achieved at a neutral pH, with 0.2 mol l^-1 H₂O₂, and 0.05 g l^-1 of NiO. However, no significant pH effects were observed in the range of 4-10. Also, the best TOC removal and Cl^- ions formation results were achieved by combined system with 48 and 80%, respectively. The illuminated NiO nanoparticles had lower influence on the degradation of 4-CP but dramatically promoted the mineralization of 4-CP. For all these reactions, the degradation rates are evaluated by determining their first-order rate constants and half-life times. The reaction rate constants ranged from 0.0003 min⁻¹ by direct photolysis to 0.029 min⁻¹ and 0.083 min⁻¹ using UV/NiO in absent and presence of H₂O₂, respectively.