In this paper the glass transition temperature (T-g) in thin polymer films has been studied. Ellipsometry has been used to measure T-g of poly(alpha -methylstyrene) (PAMS) thin films as a function of the film thickness for two molecular weights. When the films were thinner than a few hundred angstroms, substantial. reduction in T-g was apparent. The T-g depression pattern did not show the difference between molecular weights. The Michaelis-Menten equation was adapted and used to fit the experimentally obtained Tg data. From this analogized Michaelis-Menten function fitting, the fitting parameters T-g,T-infinity and xi could be obtained. The obtained T-g,T-infinity corresponded with the bulk T-g, and the parameter xi was correlated with the statistical segment length. A continuous multilayer model was proposed and derived to describe the effect of surface on the observed T-g reduction in thin films, and the depth-dependent T-g profile was obtained. These results showed that T-g at the top surface was much lower than the bulk T-g and gradually approached the bulk Tg With increasing distance from the edge of the film. The model and equation were modified to apply for the polymer coated on the strongly favorable substrate and the freely standing film.