PURPOSE: To enable to prepare a fine powder having a uniform particle size and excellent in dispersibility, in preparing the fine powder in a liquid phase, by producing and growing a fine powder under a condition stirring and suspending a plastic particle in the liquid phase.

CONSTITUTION: In 1l of an aluminum sulfate solution with concn. of 0.5mol/l, 80g plastic powder such as a vinyl chloride resin powder or the like with a particle size of 20W100μ is added and suspended therein by stirring and, after the obtained suspension is heated to 50°C, an aqueous sodium carbonate solution with concn. of 0.25mol/l is added therein to precipitate a fine powder of aluminum hydroxide. The obtained precipitate is filtered off along with the vinyl chloride resin powder and, further, washed, dried and ground to collect a mixed powder of aluminum hydroxide and the vinyl chloride resin powder. The prepared aluminum hydroxide has a uniform particle size and excellent dispersibility.