The a and 0 forms of poly(butylene terephthalate) transform reversibly by elongation and relaxation. The conformation change occurs in the tetramethylene glycol part, from GGT (GG) over bar conformation to TST (S) over barT conformation. In this study, by using a doubly oriented sample, we measured the positions, intensities, and half-widths of the (100) and (010) reflections of the alpha and beta forms of poly(butylene terephthalate) with a position-sensitive proportional counter system. During the transformation, the molecules translate only slightly. These slight molecular translations, or distortions, accumulate, and the crystallite of the alpha form breaks into the small crystallites of the beta form as the alpha-beta transformation proceeds, and the crystallite of the alpha form grows with the relaxation of the distortion accumulated in the crystal and amorphous regions and on the crystallite surface as the beta-alpha transformation proceeds.