Diazinon is one of the organophosphorus pesticides. It can threaten human and animal health, if stays in water and soil resources. Herein, the photocatalytic degradation of Diazinon under visible light was accomplished using TiO2/Fe2O3 nanocomposite, which has been never employed for diazinon degradation. Firstly, TiO2/Fe2O3 nanopowder was synthesized via ultrasonic-assisted impregnation method. To achieve this goal, TiO2 nanoparticles were effectively dispersed in the solution. Then, Fe3+ was impregnated on TiO2 nanopowder under sonication process and calcined at 300 degrees C. The prepared photocatalyst was authenticated by XRD, EDX, FESEM, TEM, PL, and DRS analysis methods yielding following results: Anatase and maghemite crystalline structure of TiO2/Fe2O3 nanoparticles, successful impregnation of Fe2O3 on the surface of TiO2 nanopowder, mean diameter size of 17 nm, extended absorption spectra of Fe2O3/TiO2 nanocomposite in the visible region, and the better inhibition of electron and electron hole recombination. Secondly, Kinetics of degradation was systematically optimized by multi variable central composite design based on response surface method. Degradation efficiency was highly affected by Diazinon dosage. The maximum experimental degradation efficiency was 95.07%. Optimum condition which was determined by software, included 0.1 g/l catalyst, 10 ppm Diazinon dosage, 45 min reaction time and 14 W/cm(2) light intensity and predicted 88.93% degradation efficiency with desirability of 0.873. (C) 2016 Elsevier B.V. All rights reserved.