Poly(ethylene terephthalate) (PET) and PET/montmorillonite(MMT) (2.5 wt%) nanocomposites with high molecular weight were prepared by solid-state poly-condensation and their fiber was spun and drawn under various conditions. The influence of MMT nanomaterials on the thermal shrinkage of PET fiber was investigated and the structure was studied using, the methods of WAXD, DSC, fiber orientation measurement, etc. The results showed that the MMT nanomaterials improved the thermal stability of microstructure of PET fiber. The fusion heat of PET/MMT was hi-her than that of PET, which generally implied the 0 high orientation or high crystallinity. However, the degree of orientation and the crystallinity of PET/MMT fiber measured by WAXD were lower than that of pure PET fiber. It is suggested that the strong interaction between MMT layer and PET restricted the motions of PET molecular chains, which developed "a special continuous network structure" and prohibited the thermal shrinkage of PET fiber. (c) 2005 Wiley Periodicals, Inc.