The phase transformation of mechanically treated gamma-Al2O3 for grinding times up to 600 min was studied. The grinding was performed under a centrifugal force, through impact and friction, at a speed of 1000 min(-1). After mechanical activation up to 120 min, the powder was heated in the temperature range 800-1100 degrees C, from 1 to 4 hours. The phase composition of samples that were only milled and of those that were milled and heated was followed using an X-ray method. The change in the specific surface area of the starting powder subjected to grinding was measured. In the case of powders mechanically treated up to 120 min and not heated afterwards, the gamma --> delta-Al2O3 transition was observed with the appearance of small amounts of the alpha phase. After 600 min of grinding the previously formed delta phase was transformed into theta-Al2O3. In the case of both mechanically treated and unground powders which were heated afterwards, the phase transition to the alpha phase could be described by the scheme gamma --> delta --> theta --> alpha-Al2O3. It developed at lower temperatures in the case of powders ground for a longer time. The kinetics of alpha-phase formation was also dependent on the time of previous mechanical treatment.