Recently, we proposed a surface-controlled cooperative rearranging region (SCC) model that directly predicts the temperature dependence of the cooperativity of segmental dynamics in super-cooled liquids. In this investigation, we employ this model to predict segmental relaxation in polymer ultrathin films. The weighted average of a hole fraction is introduced to account for the cooperativity over segments with different mobilities according to their location in the film. We evaluated the relaxation time for thin films of atactic polystyrene and demonstrated that both the glass transition temperature and the fragility parameter of free-standing and supported films decrease with a reduction of the thickness. This is qualitatively consistent with experimental observations. However, the thickness dependency obtained is weaker compared to that observed experimentally. The origin of this discrepancy is discussed.