Transparent aluminum oxynitride (AlON) ceramics have been prepared through aqueous gelcasting forming technique starting from the raw materials of single phase AlON powders. The powder was specially treated for anti-hydrolysis in ethanol before the shaping technique. The surface-treated AlON powders could then be dispersed in an aqueous-organic solution to prepare stable slurries containing 35 vol% solids loading. The obtained stable slurries were subsequently casted, calcined, and pressureless sintered at 1950 degrees C for 8h in nitrogen atmosphere. High transparent AlON ceramics with an average grain size of 112m and the in-line transmittance of 81% at wavelength 1100nm have been obtained.