The hydrolysis of aluminum nitride was increased gradually with increasing reaction time from 1 hrs to 24 hrs and/or with decreasing the addition of the reaction water from 100 mι 100mι. Amorphous aluminum hydrate, formed in the beginning of the reaction, was transformed to bayerite and to pseudoboehmite at below and above 8$0^{circ}C$, respectively. Aluminum oxynitride was synthesized by heating the partially hydrolyzed aluminum nitride at 1$700^{circ}C$ for 4 hrs or at 175$0^{circ}C$ for 30 min. AlON specimen with 1 wt% of Y2O3 that was molded and then sintered pressurelessly at 190$0^{circ}C$, exhibits 98% of the theoretical density and a translucency of 68% in the visible ray zone.