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ZnO single-crystal hexagonal microtubes were controllably fabricated by hydrothermal process. The ZnO microtubes were characterized by scanning electron microscopy, x-ray diffraction and high-resolution transmission electron microscopy. The results show that the as-prepared ZnO microtubes are hexagonal and single crystal in nature and grow on Si (111) substrates along the  direction. SEM images show that size of the ZnO microtubes are decided by ZnO sphere particles with different diameters. Studies on growth process found that the formation of the ZnO microtubes is due to the boundary equilibrium condition under high pressure in the autoclave.