The growth of Pt-Rh alloy crystallites vacuum-deposited on alpha-Al2O3 substrates by thermal treatment was investigated by atomic force microscopy (AFM) as a function of Rh concentrations. As-deposited Pt thin film without any Ph concentration changed into Pt crystallites dispersed on the substrate by annealing at 800 degrees C in oxidative and reductive atmospheres. The oxidative atmosphere remarkably enhanced the growth of Pt crystallites, while it is not the case in the growth of pure Rh crystallites. However, when Rh was added to Pt, the crystallites remained very small and highly dispersed on the substrates even after the annealing at 800 degrees C in an oxidative atmosphere. We also investigated the surface concentration of Pt and Rh by Rutherford backscattering spectroscopy (RBS) and discussed the effect of wettability of Ph oxide as a key role in inhibiting the growth of Pt-Rh crystallites.