Abstract: Nano polycrystalline diamond is a kind of material formed by direct bonding of diamond grains with grain sizes less than 100 nm. The hardness of nano polycrystalline diamond synthesized by the direct transformation of graphite at high temperature and high pressure can reach 120-145 GPa, and the hardness of the nano twinned diamond with high density twins directly transferred from onion carbon reaches 200 GPa, which is much higher than the hardness of natural single crystal diamond. Nano polycrystalline diamond has excellent properties due to its microstructures, and the investigations on the mechanism of its plastic deformation will be beneficial to the design and synthesis of nanostructured superhard materials with higher performance. In this paper, the research progress of the microstructures and the plastic deformation mechanisms of nano polycrystalline diamond at home and abroad are reviewed, and the experimental and theoretical research on the mechanisms for the plastic deformation of nano polycrystalline diamond are introduced and evaluated, which will provide reference for further research and application of nano polycrystalline diamond.