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Among wide-band-gap semiconductors ZnO is one of the promising materials for the fabrication of UV and visible light emitting devices, which recently attracted particular attention due to its remarkable optical properties.Recently the melt and hydrothermal methods have been successfully used to grow two-inch, and even larger crystals.However, there still remain serious problems with bulk-ZnO crystals.The origin of the green emission in the ZnO is the most important among them.In this paper, we investigate the growth of ZnO single crystals grown by the hydrothermal method.The crystals were processed under several annealing conditions and were characterized by photoluminescence spectroscopy (PL), Raman scattering and Electronic paramagnetic resonance (EPR) at room temperature.The origin of a green emission in undoped ZnO is discussed.