Poly(butylene terephthalate) has two crystalline forms of alpha and beta. The or form is more stable and is transformed into the beta form by stretching. The alpha<->beta transition stress was measured at various temperatures with the use of an X-ray diffraction system with imaging plates, and the relationship between the transition stress and temperature was obtained. Further, using the X-ray diffraction system, the drawing behavior of the nearly amorphous, undrawn polymer film was observed by taking up dynamically a series of X-ray diffraction patterns while a film was being drawn. The alpha form never occurred during drawing at any temperature because the stress to draw the film exceeded the alpha-->beta transition stress.