Molecular dynamics method was applied to investigate the structure and stability of γ-Al 2 O 3 surface with reference to three plane indices, namely, (100), (110), and (111). Analyses of coordination number shows that surface tetrahedral Al's were found to have more mobility than octahedral Al's. The largest change of the coordination number was observed in the (111) surface, which matches with the largest relaxation energy. The relaxation energies of (100) and (110) surfaces were found to be almost equal, and the stability of the octahedral Al's in these surfaces were compared from the analyses of the coordination numbers.