The phase transformation of mechanically treated γ-Al 2 O 3 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°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 γ→δ-Al 2 O 3 transition was observed with the appearance of small amounts of the α phase. After 600 min of grinding the previously formed δ phase was transformed into θ-Al 2 O 3 . In the case of both mechanically treated and unground powders which were heated afterwards, the phase transition to the α phase could be described by the scheme γ→δ→θ→α-Al 2 O 3 . It developed at lower temperatures in the case of powders ground for a longer time. The kinetics of α-phase formation was also dependent on the time of previous mechanical treatment.