ZnO seed grains are derived from washing ZnO-BaO sintered disks in boiling water. The grain growth of ZnO results from the energy difference across a curved grain boundary. The growth dynamics of ZnO grains and the dissolution of BaO in the boiling water are studied. The classical model shows that the growth of ZnO grains depends on grain boundary diffusion. The dissolution of BaO in boiling water can be considered as the diffusion of BaO in boiling water. The weight percent of BaO dissolved in boiling water varies linearly with the thickness of ZnO-BaO sintered disks and the reciprocal of the square root of the boiling time.