ZnO/polyaniline (ZnO/PANI) nanocomposite was produced through aniline in-situ chemical polymerization method on ZnO nanostructure. Different techniques including FT-IR, FE-SEM, EDX, DRS and TGA were employed to identify the composition and structure of the nanocomposites. The results confirmed the good quality of produced ZnO/PANI nanocomposite so that, it could be activated under UV and visible radiations. Photocatalytic activity of the nanocomposites for the degradation of metronidazole (MNZ) under UV and visible radiations as a function of time, MNZ concentration, catalyst dose, solution pH and catalyst stability was evaluated for several stages of the process. The maximum MNZ degradation (97%) under UV and visible radiations was happened over 120 and 150 min, respectively and 1.0 mg L-1 ZnO/PANI nanocomposite, 10 mg L-1 MNZ concentration at pH 7.0. In the optimal condition, kinetic studies, COD, TOC, AOS, reusability test and predicted degradation mechanism were considered in the present study. The photocatalytic activity of ZnO/PANI nanocomposite was higher than ZnO under UV and visible radiations. The constant degradation rate of MNZ by ZnO/PANI nanocomposite was 2.53 x 10(-2) min(-1), which was almost 63 times higher than ZnO photocatalysts. Besides, it was confirmed the important role of hydroxyl radicals ((OH)-O-center dot) and superoxide anion radical (O-center dot(2)-) in MNZ degradation. The photocatalytic performance under UV and visible radiations was associated with the significant absorption of UV and visible light and reduction of charge carrier recombination. With regard to the findings, it can be concluded that ZnO/PANI nanocomposite is a auspicious procedure for the removal of MNZ under UV and visible radiations. (C) 2019 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.