In this paper, aspects of the non-elastic deformation of semicrystalline poly(ethylene-terephthalate) (PET) films were studied from strain recovery and differential scanning calorimetric tests. The results show the existence of two components of non-elastic deformation, i.e. a fast-relaxing component (called anelastic) and a slow-relaxing component (usually called plastic). These strain components are both reversible and distinguished only on the basis of their different recovery times at temperatures far below the glass transition. A strain recovery master curve was built from the results of recovery tests at increasing recovery temperature. The shift-factor for the strain recovery master curve was then compared with the shift-factor for the construction of the dynamic storage modulus master curve obtained in linear regime (small strain). The aim of this comparison was to investigate the viscoelastic nature of yielding and post-yielding behavior in a semicrystalline polymer.