Treatment of effluent from dye industry by heterogeneous photocatalysis with Zinc oxide (ZnO) nanoparticle under direct solar light has been estimated. ZnO photocatalyst were prepared by sol–gel process using zinc acetate as precursor the calcination temperature at 350 °C , then characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and employed for removal of methyl orange (MO) dye from industrial wastewater. The effect of operational parameters such as pH, concentration of MO and photocatalyst dose was studied. 98.6% was the degradation percentage. The results showed that the degradation percent of methyl orange dye increased with increasing catalyst dosage and initial concentration up to 5 mg/l and 10 mg/l respectively, the acidic condition is favorable for photo-degradation of dye and better discoloration of it under UV light. Applied regression analysis model to estimate the effective and optimal parameter, the results shows a good agreement between the experimental and theoretical results. It was concluded that the photocatalytic process could be applied at large scale industrial wastewater