The thermomechanical constitutive equations are critical for shape memory polymers (SMP's) in analyzing their shape, memory, and recovery responses under different constraints. In this study, a new physical-based temperature and time-dependent constitutive model was proposed. The deformation mechanisms of this class of functional materials were explained, and the theoretical predicting values by different models were compared with available experimental results. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113; 651-656, 2009