Abstract Molecular dynamics simulations of crystalline aluminum oxide were performed for α-Al 2O 3 and γ-Al 2O 3 phases. Both bulk crystals and surfaces of each phase were studied. For each of the surfaces, several possible atomic terminations were examined and surface energies, density profiles, and atom configurations have been calculated. It was found that due to processes of surface relaxation and reconstruction some terminations of the α-alumina surfaces become more likely to appear. For γ-alumina, the occurrence of cation vacancies in the crystal structure has a significant influence on surface morphology. On the surfaces, additional active sites were observed which are not predicted by idealized models which omit vacancies.