Phase transitions in Al2O3 between corundum Rh2O3(II)-type and CaIrO3-type (post-perovskitetype) phases were examined at high pressure and high temperature in a laser-heated diamond-anvil cell (DAC) based on in situ X-ray diffraction measurements. The locations of corundum-Rh2O3(II) and Rh2O3(II)-CaIrO3 boundaries were precisely determined by conducting both forward and backward reaction experiments. The results demonstrate that corundum undergoes a phase transition to Rh2O3(II)-type structure above 106 GPa at 1800 K with a negative Clapeyron slope of –6.5 ± 1.5 MPa/K, generally consistent with earlier experimental and theoretical works. The Rh2O3(II)-type phase further transforms into CaIrO3-type above 170 GPa at 2300 K, indicating the transition pressure much higher than earlier experimental work but in agreement with reported GGA calculations. The Clapeyron slope of this phase transition was found to be high negative (–20 ± 5 MPa/K).