Abstract: A series of Pt-Al2O3 catalysts were investigated by the CS2 poisoning method. For characterizing the nature of active site, a new expression was suggested: the activity frequency was used to relate the strength of active site and the concept of number of multiplet active atoms was introduced for describing the mode of active site forming, based upon which the calculating formulas and the method of measurement were proposed. From the data obtained relating to activity frequency and the number of multiplet active atoms, it has been found that there are two types of active sites on Pt surface, designated asα- and β-site. On a-site the adsorption of CS2 is stronger than on β-site, but for the cyclohexane dehydrogenation, the β-site is more active than a-site. However it is demonstrated whether on a-site or (B)-site the dehydrogenation of cyclohexane are all proceed by the way of dual site mechanism. The role of Sn and Re all can exert influence on the catalytic properties of Pt by both structural and electronic effect.