Packing polymorphism and conformational disorder of poly(hexamethylene terephthalate) were analyzed by x-ray diffraction technique. The measurements were performed in the temperature range from 20 to 135 degrees C. At high temperature, several unassignable reflections were found to disappear, and all reflections were satisfactorily indexed by single-chain unit cell. The crystal structures of beta form (135 degrees C) and beta’ form (20 degrees C) were similarly triclinic. The beta’ unit cell assumed the double b-axis dimension, and the centrosymmetric conformations of the two chains adjacent along the b-axis differed in the orientation of the phenylene rings. At the elevated temperature (beta form), the chains were indistinguishable by xray diffraction owing to the ring-flipping motion. The beta and beta’ structures were different in the local conformational disorder around the terephthaloyl groups. Conformational poly morphism of homologous poly(oligomethylene terephthalate)s was considered to originate from the difference in bulkiness of the aromatic and aliphatic residues.