In this study, we have prepared corundum ($alpha$-Al_{2}O_{3}$) single crystals from aluminum hydroxides by hydrothermal growing process and have investigated the reaction conditions. The hydrothermal conditions were mainly affected by reaction temperature, seed crystal and reaction time. Especially, seed crystal has strong effects on the particle size and crystallity of products. By adding seed crystal in Japanese gibbsite solution as the nutrient, hydrothermal reaction was performed for 2 hours at the reaction temperature of $460^{circ}C$, to produce corundum powders which had weight mean particle diameter of $11;mu extrm{m}$ with hexagonal crystal, Without adding seed crystal in Russian gibbstite solution, corundum powders that have weight mean particle diameter of $6;mu extrm{m}$ with hexagonal crystals were also formed after 2 hours operation at the reaction temperature of $420^{circ}C$.