PURPOSE: An aluminum hydroxide aggregated particle, its preparation method, a vessel used for the method and a method for preparing an aluminum hydroxide powder are provided, to improve the transparency of a resin composition. CONSTITUTION: The aluminum hydroxide aggregated particle has an average diameter of 40 micrometers or more and an average diameter of 35 micrometers or less measured after pressing at a pressure of 1,000 kg/cm2, and the slurry obtained by mixing 20 mL of glycerin and aluminum hydroxide aggregated particle has an L value of 69 or less. The preparation method of the aluminum hydroxide aggregated particle comprises the steps of feeding a supersaturated aqueous sodium aluminate solution to a vessel; adding the aluminum hydroxide seed to the supersaturated aqueous sodium aluminate solution to form a seed-added solution; stirring the seed-added solution in the vessel while continuously feeding an additional supersaturated aqueous sodium aluminate solution into the vessel to hydrolyze the supersaturated aqueous sodium aluminate solution; separating the aluminum hydroxide aggregated particle from the aqueous sodium aluminate solution; and continuously discharging the aqueous sodium aluminate solution out of the vessel.