Aims: In this study, the adsorption process using nickel oxide nanoparticles was studied in a laboratory scale for wastewater treatment containing mono azo Orange II dye. Materials and Methods: The effect of various parameters such as initial dye concentration, pH, contact time and different concentrations of nickel oxide was investigated. The adsorption experiments were done with different concentrations of dye in a solution using a specific amount of nickel oxide nanoparticle with different pH values and a agitator speed of 100 revolutions per minute (rpm) for about 2 hours. Samples were centrifuged and the concentration of each dye was determined by ultraviolet (UV)-Vis spectrophotometer (DR 5000). Results: Based on obtained results, the optimum pH range to dye removal is acidic pH, and under lab conditions, 0.6 g/L of nickel oxide can completely remove the 50 mg/L of dye. Although increasing the dye concentration resulted in decreased process efficiency, up to 100 mg/L of dye concentration, considerable efficiency was obtained. Conclusions: The data showed that the nickel oxide nanoparticles could be used as an efficient adsorbent for decolorization of azo dyes.