Graphene-CdS (G-CdS) nanocomposite was prepared successfully via a two-step solvothermal process, with CdS uniformly dispersed on the graphene nanosheets. The photocatalytic disinfection activity of G-CdS was evaluated, and the result shows that G-CdS exhibited higher inactivation efficiency toward the gram-negative bacterium, Escherichia coli, than pure CdS nanoparticles under visible light irradiation. Exposure to 10 mg of pure CdS resulted in a 1.1-log inactivation after 60 min, but resulted in 5.3-log reduction of viable bacteria when exposure to 10 mg G-CdS under the same illumination conditions. In addition, the impacts of humic acid (HA), a kind of natural organic matter (NOM), on its bactericidal properties have also been determined. G-CdS in the presence of HA (10 mg/L) significantly decreased its toxicity, causing less than 0.4-log inactivation after 60 min. Lipid peroxidation, intercellular ROS generation and antioxidant enzyme activities assays indicate that excess oxidative stress induced by G-CdS suppressed the antioxidant defense system, subsequently affected the normal function of lipid, protein and nucleic acid, and thus resulted in cell death. However, the decreased toxicity of G-CdS in the presence of HA may be attributed to that (1) HA presents a barrier to prevent the physical contact between bacteria cells and G-CdS, which was the first step of toxicity mechanisms; (2) HA acts as an antioxidant to react with any ROS and reduce the toxicity. (C) 2015 Elsevier B.V. All rights reserved.