The atomic and electronic structures of the liquid Al/(0001) α-Al(2)O(3) interfaces are investigated by first-principles molecular dynamics simulations. Surprisingly, the formed liquid-solid interface is always atomically abrupt and is characterized by a transitional Al layer that contains a fixed concentration of Al vacancies (~10 at.%). We find that the self-regulation of the defect density in the metal layer is due to the fact that the formation energy of the Al vacancies is readjusted in a way that opposes changes in the defect density. The negative-feedback effect stabilizes the defected transitional layer and maintains the atomic abruptness at the interface. The proposed mechanism is generally applicable to other liquid-metal/metal-oxide systems, and thus of significant importance in understanding the interface structures at high temperature.