SYNOPSISThermal 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 welloriented 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.