The effect of humidity on crystallization and polymorphic transformation of hydrous aluminum oxide under various humidity at 37° was examined by means of X-ray diffraction, scanning electron micrograph, IR spectra and DTA. The humidity was an important factor influencing crystallization of hydrous aluminum oxide. The growth of crystal was strongly accelerated by humidity. The aging process is assumed that it is composed of two seperate steps, an increase of the diffraction around 36∼42°, and an appearance and its development of the peak at 18∼20° of 2θ value. The former is considered to be nucleation and the latter correspond to the growth period on crystallization. The crystalline form of aging products was various depending on the degree of humidity, directly it leads to the eventual formation of bayerite in more than 72%, bo¨hmite in 50% and resembled to Nordstandite in 0% relative humidity, respectively but once formed, it was mostly stable in each surroundings and does not transform to the other more stable form in solid state even after aging for five years. The mechanism responsible for aging is further polymerization process of six-membered rings by deprotonation-dehydration reaction in which positively charged polynuclear hydroxy aluminum complexes formed in the presence of moisture are joined at their edges by double hydroxide bridges.