Abstract: Nitrogen-doped graphene quantum dots (N-GQDs) were synthesized by the hydrothermal method, using citric acid and urea as the carbon and nitrogen sources, respectively. X-ray diffraction, Raman spectroscopy, transmission electron microscopy, fluorescence spectroscopy and UV-visible absorption spectroscopy were used to characterize their microstructure and photophysical properties. The cytotoxicity of the N-GQDs was tested using 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide. The photocatalytic degradation activity for methylene blue (MB) was investigated under visible light. Results show that the N-GQDs have a narrow size distribution, a high fluorescence and a low cytotoxicity. Photocatalytic degradation rate of the N-GQDs for MB reaches 82.5% under visible light irradiation for 120 min.