Aluminum nitride was hydrolyzed in contact with water, evolving the reaction heat of 172 cal/g within 12 hours to form alumina trihydrates. At 4$0^{circ}C$ >, amorphous alumina hydrate was easily produced by the spontaneous breaks of AlN particle at the beginning of the hydrolysis process, while bayerite was formed by the dissolution-recrystallization processes of amorphous alumina hydrate at the temperature between 4$0^{circ}C$ and 6$0^{circ}C$, and pseudo-boehmite was generated on the surface of AlN particle by the condensation process of the corresponding phase at 6$0^{circ}C$ <. The longer the hydrolysis timje or the higher the value of pH in solution, the more the bayerite phase was produced. However, pseudo-boehmite was easily generated under the following favorable conditions; when the hydrolysis reaction occured rapidly at the beginning and when the absorption of OH radical on the surface of AlN particle was disturbed by ethyl alcohol in a solution. However, aluminum nitride was hardly hydrolyzed in a solution of pH 2.0