Grain fragmentation and property modification of nanocrystalline ZnO under high pressure were studied on CS-1B 6×8000 kN cubic high pressure apparatus. Grain size and microstructure of the samples have been characterized by X-ray diffraction and field emission scanning electron microscopy. The results show that the significant grain fragmentation phenomenon occurs inside the crystallites. Experimental results on hardness and the current-voltage characteristics indicate that micro-hardness of the samples after high pressure is 2.3 times that of the samples sintered at atmospheric pressure and the nonlinear current-voltage characteristics is obviously better than that of the latter.