Thermal treatment of gibbsite (Al(OH)(3)) powders of different particle size was studied in inductively coupled thermal plasma operating at normal pressure. Calcination and spheroidization took place simultaneously although spheroidization did not occur with coarse feed particles. Microsized or nanosized Al2O3 particles of different phase composition were produced depending on the size of injected grains. The microsized powders were mainly formed by melt solidification. In the formation of nanopowders, however, evaporation of small grains followed by homogeneous nucleation from the vapor phase and grain growth seem to be the dominant processes.