PURPOSE: To produce twin alumina particles suitable for use as an abrasive material, starting material for a porous ceramic sintered compact, a filler for a coating material, etc.

CONSTITUTION: Aluminum hydroxide or alumina hydrate whose particle size has been regulated to a submicron order is hydrothermally treated as starting material with water or an aq. alkali soln. At this time, the ratio of the starting material to the water or soln. is regulated t.o 1-10 and the objective twin alumina particles are produced. Each of the particles consists of flat platy alumina crystals 1, 2, 3 grown in a penetrating and crossing state.