ZnO nanocrystals were prepared by precipitation method. After aging, washing and drying, solids were calcined at 300 degrees C and examined as photocatalyst for UV-induced degradation of organophosphorus insecticide diazinon in aqueous solution. The average crystallite size of prepared ZnO nanopowder was determined from XRD pattern using Debeye-Scherrer equation in the range of 14 nm. The experiment showed that ZnO nanopowder and UV light had a negligible effect when they were used on their own. The effects of some operational parameters such as pH value, nanocatalyst loading and initial insecticide concentration on the degradation efficiency were discussed through the photocatalytic experiments using prepared ZnO nanocrystals as the photocatalyst. In this case the optimized parameters were also tested for the treatment of real water containing the insecticide. A model was successfully established for the prediction of degradation of diazinon in UV/ZnO process with any concentration of insecticide. Catalytic activities of prepared and commercial ZnO nanocrystals were also compared. The diazinon photodegradation quantum yield in UV/ZnO process increased with decreasing the diameter size of ZnO from 33 to 14 nm. The results showed that the photocatalysis process in the presence of ZnO with mean size of 14 nm offered the best energy efficiency. Accordingly, it could be stated that 80% removal of the insecticide, after selecting desired operational parameters could be achieved in a relatively short time, about 80 min. (c) 2007 Elsevier B.V. All rights reserved.