Shape memory polymers (SMPs) can keep a temporary state and subsequently recover to the original shape through a prescribed thermomechanical process. Although different theoretical models have been presented, the viscous effects were seldom considered. This article aims to provide an insight into the viscoelastic property of SMPs and its effect on the functional realization. Systematic thermomechanical experiments were performed. Special considerations were focused on the viscoelastic response of SMPs in the vicinity of the glass transition temperature T-s. The relations between shape switching transition temperature T-tran and T-g were also discussed. The results confirm that T-tran departs from T-g due to the viscoelastic effect and does not keep a constant value during heating and cooling processes. The viscoelastic effect reaches to maximum value at T-g, then decreases slowly at cooling and quickly at heating. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 1406-1413, 2010