Dynamic mechanical analysis was successfully used, for the first time, to characterize polymer thin and ultrathin films supported on substrates. This method allowed us to uncover the effects of the free surface and the substrate interface on the segmental dynamics in polystyrene (PS) films with various thicknesses. As the film thickness decreased, the distribution of relaxation times for the segmental motion became broader, a change mainly due to surface and interfacial effects. Interestingly, PS ultrathin films sandwiched with SiOx layers exhibited a relaxation process corresponding to the interfacial segmental motion in addition to that in the internal region of the films. The results obtained in this study imply that two contrasting effects exist: the effect of the free surface accelerates the segmental motion, whereas interfacial interactions produce the opposite effect.