In this research, treatment of wastewater containing azo dye of acid orange 7 was investigated. According to low capability of conventional photocatalytic process in treating colored wastewater in comparison to ozonation, and high ability of ozone in discoloration, system of ozonation/ photocatalytic processes was used for treatment. Nano TiO2 was used as photocatalyst. The effect of pH (3, 5, 7, 9) and initial concentration (5, 35, 50 mg/L) on removal efficiency of organic pollutant was investigated by ozonation/photocatalytic system and phtocatalytic process. An experimental design based on the response surface method was applied to assess the optimum level of operating parameters on the treatment energy consumption in textile industry wastewater. Based on experimental design data, optimum conditions were achieved at 2.95 day of retention time, pH of 5, ozone injection rate of 1.2 g/h, 128 mg/L of nano TiO2 and 120 Wt UV-A light source.