Highly blue-luminescent nitrogen-doped graphene quantum dots (N-GQDs) are obtained by hydrothermal treatment of graphene oxide in the presence of ammonia. The yield of N-GQDs is about 8.7% in weight. A high quantum yield of maximum 24.6% at an excitation wavelength of 340 nm is achieved. They are applied for bioimaging of HeLa cells, and showed bright luminescence and excellent biocompatibility.