Bulk single crystals of ZnO were grown by the hydrothermal method using a mixture of KOH and LiOH aqueous solution. These crystals were transparent, pale green or pale yellow in color depending on the growth conditions. The optical property of these sectors was studied by cathodoluminescence (CL). In the + c region (grown with Zn surface), strong green emission (2.3 eV) was observed together with the UV emission. Tn the - c region (grown with O surface), weak orange emission (2.1 eV) and weak UV emission were observed. Tn the m region (prism surface), the UV emission was the strongest while the orange emission was weak. These results indicate that the m region is preferable to grow good crystals for the UV application. Origins of these visible luminescence were discussed from the viewpoint of the crystal structure and growth mechanism.