Planar optical resonant structures with high quality (Q) factors play a crucial role in modern photonic technologies. In this paper, a type of remarkably high-Q resonant nanostructure based on atomically thin two-dimensional (2D) materials is proposed. It is shown theoretically and numerically that with the excitation of leaky modes in the proposed structures, guided mode resonant (GMR) gratings, can achieve resonances with extremely narrow linewidths down to 0.0005 nm and high Q-factors up to millions in the telecom range. The thickness of 2D materials and thus the high-Q resonances can be precisely controlled by changing the layer number of 2D materials, providing a versatile platform for strong light-matter interactions. As an example, dramatic nonlinear reflectance can be realized around the resonance at a power level of a few kW cm-2 with the Kerr effect. This new type of 2D material resonant nanostructure can be employed for a variety of applications ranging from lasers, filters and polarizers to nonlinear optical devices.