An experimental study of film formation and structure development of polybutylene terephthalate (PBT) in single and double bubble tubular film extrusion processes is presented. PBT was found largely stable in the first stage. In the second stage, the bubble was unstable and biaxial inflation was achieved only with a blow-up ratio of one. The sources of second bubble instabilities were related to the crystalline nature of the first bubble being inflated in the second stage. The structure and properties in the films varied substantially with processing conditions. While the first bubble had poorly ordered alpha-structure, the double bubble possessed oriented polymorphic texture. The second deformation that induced high film-line stress favored the formation of a stable beta phase. The mechanical behavior of the films reflected the crystal perfection as well as orientation in the films.