The effects of Strain Induced Crystallization and chain orientation on the cooperativity evolution with temperature are studied on two polymers with similar molecular structure but different crystallization attitude. For this goal, we focus on poly(ethylene terephthalate) (PET) and the copolyester poly(ethylene glycol-co-cyclohexane-1,4-dimethanol terephthalate) (PETg), the last one having very low ability to crystallize. From Temperature Modulated Differential Scanning Calorimetry and Broadband Dielectric Spectroscopy investigations, we show that the crystalline phase appearance in PET implies a large reduction of the Cooperative Rearranging Region size, accompanied with a variation from fragile to strong behavior of the structural relaxation time temperature dependence. Such large variations are not observed in PETg at the same draw ratio. In the case of PETg, a small contribution of chain alignment to the CRR size evolution is evidenced. (C) 2014 Elsevier Ltd. All rights reserved.