Dissolved Organic Nitrogen (DON) is a known precursor for carcinogenic nitrogenous disinfectant by-products (N-DBPs). However, few effective methods were found to remove DON from water. In the paper, removal efficiency and mechanism of DON derived from Microcystis aeruginosa cells by the sunlight/Cu-TiO2 and sunlight/TiO2 was investigated. In addition, the effect of the oxidation process to the coagulation was also investigated. The results showed that there were about 15 main protein categories in the water samples derived from M. aeruginosa cells, the concentrations of which were higher than 0.066 mg/L. The removal rate of DON in the sunlight/Cu-TiO2 system is 39.2% after 7 h, which is 10% higher than that of sunlight/TiO2 system (29%). During the oxidation process of sunlight/Cu-TiO2 system, soluble protein and the concentration of Total Nitrogen (TN) decreased significantly, and high molecular weight proteins (HMWP) were degraded into low molecular weight proteins (LMWP). The photocatalytic oxidation could significantly enhance the removal effect of DON by coagulation in the first 1 h, following by an obvious decreasing. (C) 2015 Elsevier B.V. All rights reserved.