Background & Objectives: Pollution of groundwater and surface waters to nitrate is an environmental problem in many parts of the world including Iran. It might cause diseases such as methemoglobinemia, lymphatic system, and blood cancer. Therefore, nitrate removal from water resources is necessary. Since application of nanomaterials in treatment of the environment pollutants has become an interesting method, the objective of this study was application of ZnO nanoparticles under UV radiation to remove nitrate from aqueous solution. Methods: Three nitrate concentrations of 50, 100 and 150 mg/l were considered for present study. In order to determine the optimum level of ZnO nanoparticles on nitrate removal, concentrations of 0.1, 0.4, and 0.8 g/l were used. PH ranges of 5-9 were studied. Also, the effect of UV irradiation was investigated. Results: In this study, the effects of exposure time, nitrate concentration, ZnO nanoparticle dose, and pH were investigated. The results showed that nitrate removal decreases with increasing initial nitrate concentration. Also, nitrate removal efficiency increased from 75.7% at 0.1 g/l to 91.2% at 0.8 g/l 0f photo-catalyst. Maximum reduction efficiency was observed in neutral pH. For UV application, maximum performance of 27% was observed at pH=9. Conclusion: Application of UV irradiation solely did not effectively removed nitrate from aqueous solution, however, efficiency of combination of ZnO/UV process was appropriate for nitrate removal.