The evolution of structure and properties in uniaxially stretched polybutylene terephthalate (PBT) films has been explored. The stretch temperature has pronounced influences on the development of PET polymorphism; the mechanisms of its formation involve levels of both applied stress as well as chain relaxation during the stretch period. The stretch rate, on the other hand, tends to perfect the structure. PET films produced under different conditions develop different levels of crystallinity and orientation in the individual alpha and beta phases, in which annealing under tension induces a partial crystal transition from the beta to alpha phase that leads to an increment of orientation in the alpha phase. The mechanical behavior of the films is related to the polymorphic structure as well as to the overall chain orientation.