The growth behavior of Pt crystallites vacuum-deposited on flat gamma- and alpha -Al2O3 substrates was investigated by atomic force microscopy (AFM). In this study, we fabricated a thin layer of gamma -Al2O3 on a planar sapphire (single crystalline alpha -Al2O3) substrate by Al+- and O+- ion implantation and annealing at 900 degreesC. AFM observation revealed that gamma-and alpha -Al2O3 substrates have a negligible roughness compared with the size of the supported Pt crystallites deposited on them. The crystal structure was identified as a gamma (111) plane for the gamma -Al2O3 substrate, but an alpha (0001) plane for the sapphire substrate by Rutherford backscattering, spectroscopy-channeling analysis (RBS-C) and X-ray diffraction analysis (XRD). We then observed the surface morphology of Pt crystallites on gamma- and alpha -Al2O3 substrates, which were fabricated by sputtering of a Pt target. The as-deposited Pt thin film changed into Pt crystallites dispersed on the substrate with heat treatment at 800 degreesC in an oxidative atmosphere. It was found that the growth of Pt crystallites on gamma -Al2O3 was less, although the particle growth of Pt on alpha -Al2O3 was remarkable. The growth process of the Pt crystallites was also discussed along with the surface structure of the Al2O3 substrates. We attribute the inhibition of Pt crystallite growth on the gamma -Al2O3 substrate to the defective structure of,the gamma -Al2O3 compared with the stable structure of gamma -Al2O3.