A model was developed to quantify the thermal shrinkage behavior of polyester shrink film under isothermal and nonisothermal conditions. This model employs the straight forward first order reaction kinetics to an otherwise microscopically complex relaxation process. It offers three unique parameters for quality control and material specification of shrink film. In the isothermal condition, the induction time, the shrink constant, and the ultimate shrinkage are the three parameters used by the model to quantify the shrinkage of shrink film under an elevated but constant temperature. In the nonisothermal condition, the shrink film is heated using a constant heating rate. Three parameters, which are the onset shrink temperature, the shrink constant. and the ultimate shrinkage. characterize the shrinkage of shrink film. The model fits the experimental data very well. The thermal shrinkage of several polyester homopolymers, copolymers, and polyester blends was investigated and compared to that of poly(vinyl chloride) (PVC) shrink film. Two polyester blends containing glycol modified poly(ethylene terephthalate) (PETG) copolyester behave similar to PVC in shrinkage behavior. The effect of stretch temperature, stretch ratio, and stretch rate on the shrinkage characteristics of oriented PETG film was also studied. The stretch temperature has the highest influence on the shrinkage behavior of oriented film.