Thermal stability is a critical issue for polymer thin films being used as interlevel dielectrics (ILDs) in deep- submicron multilevel interconnection. One of crucial parameters to predict thermal stability is the glass transition temperature (Tg). Unfortunately the glass transition in polymer thin films is still not clearly understood. In this work, a simple model is developed for the thickness dependence of Tg of polymer thin and ultrathin films. It is predicted that Tg of polymer thin films can either be reduced or enhanced in comparison with its bulk values, depending on the polymer-substrate and polymer-surface interactions. In addition, the thickness- dependent Tg of polymer thin films can exhibit a minimum as a function of thickness. Experimental data from technologically important ILD films are obtained to support the theoretical model.