To improve the long term stability of alumina sol to be used as powder or thin film, and to lower the thermal treatment temperature for powder and thin film of α-alumina.

The alumina sol is successfully stabilized for a long period of time by using an alkoxy diglycol as the solvent instead of water, aluminum isopropoxide as the raw material and by adding acetic acid as the additive. By using this alumina sol, powder or thin film of α-alumina can be formed at a low thermal treatment temperature of ≥700°C, industrially 950°C.