It is well known that the intrinsic melting mechanism is independent of crystal size according to Lindemann's melting criterion. In order to probe whether the glass transition mechanism is also size-independent, segment dynamics of free-standing polystyrene (PS) films is determined by considering the temperature- and rhickness-dependent number of styrene segments N-alpha(TD) in the cooperative rearranging region (CRR). Under the help of Adams-Gibbs glass transition theory and molecular dynamics simulation, N-alpha(TD) function is established and it decreases as D decreases or T increases. However, N alpha[T-g(D),D] at the glass transition temperature Tg(D) is size-independent, which is consistent with the simulation results obtained by Donth's method. Meanwhile, its relative temperature function N-alpha[T-T-g(D)]/T-g(D)) is also size-independent. Therefore, N alpha[T-g(D),D] function as a criterion for glass transition, which describes the physical nature of the glass transition, is similar to the vibrational amplitude in Lindemann's melting criterion. (C) 2008 Elsevier Ltd. All rights reserved.