Ethane dehydrogenation (EDH) over Pt catalysts represents a promising technique for the production of olefins, however atomic-level understanding into the factors controlling catalytic performance of this process is still elusive. Herein, a DFT based micro-kinetics model is established to address the role of the active site, the catalyst composition and the reaction conditions on the C2H4 activity and selectivity from EDH. Undercoordinated step sites are found to have higher EDH activity than close-packed terrace sites. In addition, alloying Pt with Sn dramatically increases C2H4 selectivity by suppressing all other side-reactions. Our results provide a comprehensive understanding of observed experimental phenomenon during EDH, which is the basis for intelligent design of catalysts for alkane dehydrogenation reactions.