Hexagonal diamond nano-grains in irradiated surface of graphite produced by ion bombardment at high doses of 0.6keV Ar+ have been found,using high-resolution transmission electron microscopy (HRTEM).The average diameter (D) of the grains was varied between ～2 and ～50nm,and the grain population decreases almost monotonic ally as grain size increases.It is of interest to note that they exhibit a distinctly bimodal structure.For diameters less than 10nm,the microstructure of grains are single crystals,but the polycrystal structure of nano-grains with larger diameter (>10nm) can be also observed.The characteristics of this bimodal structure are on the basis of hexagonal diamond nano-grain nucleation and growth mechanisms,which are restricted by the characteristics of ion bombardment processes.