The formation mechanism of a high density of nanovoids by annealing amorphous Al 2 O 3 thin films prepared by an electron beam deposition method was investigated. Transmission electron microscopy observations revealed that nanovoids $1-2 nm in size were formed by annealing amorphous Al 2 O 3 thin films at 973 K for 1-12 h, where the amorphous state was retained. The elastic stiffness, measured by a picosecond laser ultrasound method, and the density, measured by X-ray reflectivity, increased drastically after the annealing process, despite nanovoid formation. These increases indicate a change in the amorphous structure during the annealing process. Molecular dynamics simulations indicated that an increase in stable AlO 6 basic units and the change in the ring distribution lead to a drastic increase in both the elastic stiffness and the density. It is probable that a pre-annealed Al 2 O 3 amorphous film consists of unstable low-density regions containing a low fraction of stable AlO 6 units and stable high-density regions containing a high fraction of stable AlO 6 units. Thus, local density growth in the unstable low-density regions during annealing leads to nanovoid formation (i.e., local volume shrinkage).