Grain evolution of nano-crystals ZnO under high temperature and pressure is studied using a cubic high pressure apparatus. The structure, grain sizes and morphology of the samples are characterized by X-ray diffraction and field emission scanning electron microscopy. The results show that the grain sizes of ZnO grow rapidly at temperature 200℃ under pressure. At temperature lower than 300℃ (including 300℃), the grain sizes of the samples first increase with the pressure increasing from 1 to 3 GPa and later decrease from 4 to 6 GPa. The activation volume from 1 to 3 GPa and from 4 to 6 GPa is calculated respectively using the phenomenological kinetic grain growth equation at temperature 300℃. At temperature higher than 400℃ (including 400℃), the grain sizes of the samples increase with the pressure increasing from 1 to 6 GPa. ZnO nano-bulks with good quality can be obtained under the specific conditions.