A facile one-step hydrothermal process is presented for fabrication of graphitic carbon nitride/carbon nanodots (g-C3N4/C-dots) composite photocatalyst. XRD, SEM, TEM, UV-Vis DRS and PL were adopted to analyze the effect of C-dots modification on the structure and properties of g-C3N4. Photocatalytic activity of g-C3N4/C-dotswas evaluated by the degradation ofmethyl orange (MO) under different conditions. The results show that C-dots can increase photocatalytic reaction rate of g-C3N4/C-dots composite under visible-light irradiation, especially in the presence of H2O2. C-dots act as electron-sinks, therefore preventing the recombination of photo-generated electron-hole pairs. Meanwhile, H2O2 not only acts as an oxygen-donor to make contribution to generation of O-2, but can directly react with the photo-generated electrons transferred from the conduction band of g-C3N4 to C-dots, leading to the formation of center dot OH active group, which promotes the degradation of MO. Therefore the photocatalytic activities of g-C3N4/C-dots can be improved significantly when adding H2O2 to the photocatalytic reaction system. (C) 2017 The Electrochemical Society. All rights reserved.