This paper reports a facile fabrication of N-doped graphene quantum dots (N-GQDs) showing controllable chemical properties through a hydrothermal treatment. The N-GQDs have a uniform size of 3.06 +/- 0.78 nm and prefer the equilibrium shapes of circle and ellipse due to the minimization of edge free energy. The N/C atomic ratio in N-GQDs can be precisely tailored in a range from 8.3 at% to 15.8 at% by simply controlling the concentration of N source (ammonium hydroxide). One order of magnitude quantum yield of 34.5% is achieved by N-GQDs, compared with the N-free GQDs, as the substitutional N has an essential role in more effective radiative emission. Excessive N dopants in N-GQDs can lead to photoluminescence quenching, through nonradiative transition back to the ground state. The N-GQDs are further found to be suitable as photocurrent conversion materials due to benign energy matching with anatase nanofibers, the ultrafast electron injection at their interface, and efficient electron transfer. This work provides an efficient and inspiring approach to engineering both chemical components and physical properties of N-GQDs, and will therefore promote their basic research and applications in energy conversion.