Thermal shrinkage, viscoelastic properties, wide- and small-angle X-ray diffractions, and molecular orientation by fluorescence method were measured for fiber samples of nylon 46 [poly (tetramethylene adipamide)] as compared with nylon 66 [poly (hexamethylene adipamide)]. The structural study of nylon 46 fiber indicated that molecular chains are well-oriented along the fiber axis in the interlamellar region whose thickness is relatively thin. It was also shown that the onset-of thermal movement of nylon 46 molecules is shifted to the higher temperature than nylon 66. These characteristics will be attributed to the cause of good dimensional stability, a significant feature of nylon 46 fiber, attractive for its application to industrial usages.