A recently developed high coordination lattice based Monte Carlo method is applied to investigate the glass transition of free-standing atactic polypropylene thin films. In this method the short-range interactions are described by a rotational isomeric state model and the long-range interactions are obtained from a discretized form of a Lennard-Jones function. Simulations of five thin films of C150H302 with thickness of 20-100 A at T-g are performed by a stepwise cooling from the melts to the glass states. The temperature dependence of the thickness of the thin films is obtained from the fitting of the density profile by a hyperbolic function. The glass transition temperatures of the thin films are evaluated from the change in thermal expansion coefficients at T-g. The T-g decreases as the films become thinner. For all but the thinnest film, 1/T-g is inversely proportional to the thickness of the film. The proportionality constant is within the range cited in experimental studies of other systems. The depression of the glass transition temperature arises mainly from the fast dynamics in the surface region. (C) 2003 American Institute of Physics.