Solid-state electronic spin system of the nitrogen-vacancy (NV) center in diamond is attractive as a nanoscale quantum sensor under room-temperature dueto its unique characteristics such as stable fluorescence, long coherent time, and near-atomic size under ambient conditions. Nowadays, the NV center plays a significant role in super-resolution microscopies. Different super-resolution microscopies have been used on NV center to archievenanoscale spatial resolution. Moreover, the spin state in NV center can be regraded as a solid-state qubit, which can be optically polarized and read out. The spin state can couple with electromagnetic fields and strain, which enables the NV center to be an excellent quantum sensor with high spatial resolution and high sensitivity. Such an NV-center based quantum sensing technique is being developed for applications in newmateriales, single protein nuclear spin dynamic field, life science, etc. This review will introduce the basic principle of such a nanoscale quantum sensor, the experimental realization, methods of enhancing the sensitivity, and some applications in high-spatial-resolution and high-sensitivity sensing.