Abstract: To investigate the role of steam on the structure and activities of platinum-based catalyst, a tin modified Pt/gamma-Al2O3 catalyst was prepared for propane dehydrogenation in the presence of steam. The propane conversion could be prominently elevated by nearly twofolds with only 1.0% loss of selectivity to propene when an optimum amount of H2O was added. The retarding effects of steam on the activities are also observed at higher H2O/C3H8 molar ratios (>1). Kinetic studies results show that the apparent activation energies for steam dehydrogenations are lower than those without steam addition. The fresh, spent and steam pretreated catalysts were characterized by TG, Raman, H-2-chem, HAADF-STEM, XPS and CO adsorption FT-IR (CO-FTIR). It is observed that the platinum particles sizes are smaller in the samples subjected to certain period of steam pretreatment. The changes of platinum dispersion in steam conditions are related to the changes of tin oxidation states and the segregation of platinum from Pt-Sn alloys, as revealed by CO-FTIR and XPS results. More accessible Pt sites that are generated in the steam due to the phase transformation of Pt-Sn catalyst as well as the removal of coke accumulated on the catalyst surface would account for the increase of catalytic activities and the decrease of activation energies in steam atmosphere. Competitive adsorption of H2O and other reagent over Pt sites would lead to the retarding effects of steam on the activities at high H2O partial pressure.