The glass transition temperature (T-g) in the thin poly(alpha-methyl styrene) and polysufone films that are coated on the surface of a Si wafer is investigated in this paper. A novel. equation was suggested to estimate the thickness dependence of T-g in thin films. Ellipsometry has been used to measure T-g of various thin films as functions of sample thickness, molecular weight, and kind of polymer. One adjustable parameter in the equation, i.e., xi, was obtained from fitting this function with the experimentally measured T-g. The magnitude of the fitted parameter xi showed discrepancies between the polymer types but similarity between the molecular weights. This parameter xi corresponded to the statistical segment length of the polymer. All the measured T-g data of the investigated polymers were superimposed in one curve by normalizing the thickness and the measured T-g with the parameter xi and bulk T-g, respectively. The origin of T-g depression phenomena in thin films was closely correlated with its segmental length and not with whole chain size.