In situ room temperature scanning tunneling microscopy STM observations combined with low energy electron diffraction and Auger electron spectroscopy were performed to investigate the structure of a thin aluminum oxide film grown on Ni111. Well-ordered alumina films were obtained after the deposition of 2.5 ML of aluminum on a clean Ni111 surface, followed by its oxidation under O2 flow and subsequent annealing at 1000 K. Whereas an hexagonal unit cell corresponding to a 5353 reconstruction with respect to the Ni111 surface had been previously ascribed to this superstructure, our results indicate that the unit cell corresponds to a sixton rectangle, i.e., a rectangle with a 3 ratio between the lengths of the two sides of the mesh 18.210.5 Å2. We attribute this specific ratio to the presence of the hexagonal arrangement of an oxygen plane in the layer. From the size and aspect ratio of the mesh and from the STM observations, we also conclude that the atomic organization observed for alumina/Ni111 is very similar to the organization observed for alumina grown on FeAl110, NiAl110, Cu-9 at. % Al111, and Cu111, which provides strong argument that this alumina structure is not specific of aluminum-based substrates but could be the equilibrium state of a two-layers-thick alumina film on a metal.