Aluminum hydroxides were synthesized by a simple electrolytic reaction of aluminum plates. The aluminum monohydroxide, boehmite(AlO(OH)), was predominantly formed by the application of an electrical potential above 30V, while the mixture of the bayerite(Al(OH)3) and boehmite(AlO(OH)) phases were formed below 20V. The boehmite has a clear fibrous structure which is controlled on a nanometer scale. On the contrary, the bayerite consists of the typical hourglass or semi-hourglass shaped coarse crystals as a result of an aggregation of the various crystals stacked together. The specific surface area of the boehmite nanofiber was remarkably high, reaching about 300m2/g.