A discontinuous volume change was observed on the Hugoniot curve of corundum (&agr;-Al2O3) along the ⟨a⟩ axis by the inclined-mirror streak photographic technique, probably due to a high-pressure phase transformation. The transition pressure was measured to be 79 GPa, where shock temperature was estimated to be 860 ¿C, in good agreement with a fully optimized quantum mechanical calculation. The volume change at the transition point was considered to be larger than 3%, which was larger than the theoretically predicted value of the corundum-Rh2O3 (II) transition, and was consistent with the recent static compression result by using diamond-anvil cell. The occurrence of the phase transformation in alumina under shock compression presents significant implication in earth interior science or ceramic science, and also seriously affects the interpretation of static and shock compression experiments at ultra-high pressures.