Aluminium is produced from bauxite ore [1]. Gibbsite [Al(OH)3], boehmite [γ-AlO(OH)] and diaspore [α-AlO(OH)] that are aluminium hydroxide minerals [2] can be found in bauxite ore at different ratios. Aluminium is only produced from gibbsitic bauxite by Bayer process. Beside the leaching conditions, the type of bauxite ore is the key factor of the production. While ores with high gibbsite content can be processed at about 140°C, boehmite requires 200–240°C and stronger caustic solutions, higher temperatures and pressures are needed for complete extraction from diasporic bauxite [3]. In this study, diasporic bauxite was mechanically activated for different durations (15, 30, 60, 90 and 120 min) by planetary mono mill. Speed of main disc and ball-to-mass ratio were kept constant at 600 rev.min-1 and 25 respectively. Structural changes and thermal behaviour were examined by XRD, SEM, DTA and FTIR. Diaspore and hematite were detected at XRD analysis and with increase in activation time, intensity of the peaks decreased and position and width of the peaks changed as well. In DTA analysis, with mechanical activation, temperature of the endothermic peak that corresponds to dehydroxylation of the aluminium hydroxide, was lowered and area was decreased. These results mean that the structure was disordered with mechanical activation.