We have studied thermal expansion of free standing polystyrene thin films using X-ray reflectivity to elucidate the glass transition temperature and the thermal expansivity. We found that the glass transition temperature T(g) decreased with the film thickness, depending on molecular weight. The reduction in the free standing films is much larger than in the supported films on Si substrate, suggesting that some segmental motions are activated due to free surfaces on both sides in the free standing films. We also found that the thermal expansivity in the glass and the melt decreased with the film thickness. This decrease must be attributable to chain confinement effects.