An experimental study of structure development in cast film extrusion and biaxially stretched films of polybutylene terephthalate is presented. Cast films exhibited only alpha-form crystallites and relatively low crystallinities (X-c= 3.5-14%), while biaxially stretched films showed a mixed character of both alpha- and beta-form crystallites with crystallinities of 20-30%. Crystal perfection increases with uniaxial deformation. Biaxiality causes a decrease both in crystal perfection and crystallinity of the films. Annealing increases crystallinity. The orientation of the films has been determined by refractive indices and WAXS pole figures. Polymer chains orient rapidly to the machine direction (MD) with uniaxial stretching. Increasing biaxiality causes an increase in population of TD-oriented crystals. The planarity of the film increases with simultaneous transverse stretching. The phenyl rings orient into the plane of the film. Annealing further increased the orientation of the films.