When polyethylene terephthalate (PET) is stretched, it exhibits strain-hardening properties, which are temperature and strain-rate dependent. In this paper, two grades of PET are experimentally characterized using biaxial tests. A visco-hyperelastic model is used to describe the stretching behavior for the polymer. A biaxial characterization method is employed to determine the model parameters using a robust nonlinear curve-fitting program. This model can represent adequately well the stretching behavior of PET. Based on this model, the membrane finite element formulation is developed to simulate the stretch blow molding process. Two bottles of different designs, produced based on the single-stage injection blow molding process, are used to validate the model. Good agreement with the bottle thickness profile is observed. (C) 2004 Society of Plastics Engineers.