The glass transition temperature (Tg) of poly(α-methyl styrene) (PαMS) films supported by silica is studied as a function of film thicknesses from ~17 to ~168nm at three molecular weights of 1.3, 20 and 420kg/mol. For the 20 and 420kg/mol films, the glass transition temperature decreases with decreasing film thickness, consistent with previous results. But for the 1.3kg/mol films, it becomes independent of the film thickness. We tentatively suggest the Tg depression to be caused by free volume excess at the polymer–air interface and that its influence diminishes at low enough molecular weights because of a chain stiffness effect.