Abstract: Catalytic propane dehydrogenation over an alumina supported Pt catalyst in the presence of steam is carried out and it is found that the catalyst activity is increased and the apparent activation energy is lowered due to the presence of steam. Three possible mechanisms, i.e. co-adsorption, Langmuir-Hinshelwood and Eley-Rideal, of changes in energetics and pathways for propane dehydrogenation due to the presence of steam are explored by DFT calculation. The results show that co-adsorption of C-3 species with surface oxygenated species would elevate dehydrogenation energy barriers due to repulsive interactions between them. Surface -OH is more active than surface -O in activating the C-H bond in propane and propyl species through either the Langmuir-Hinshelwood or Eley-Rideal mechanism and plays an important role in propane dehydrogenation with steam. The Langmuir-Hinshelwood mechanism is kinetically favorable, in which the activations of the first H in propane by surface -OH are the rate determining steps, but the activation energies are higher than that on a clean Pt(111) surface. The observed enhanced catalyst's activity is ascribed to the lowered coking rates as well as the changes in surface coverage due to the co-adsorption of water and the surface oxygenated species.