Nylon 6 and 6.6 fibers were submitted to thermal annealing in a wide range of temperatures (below and above their glass transition temperatures) under inert atmosphere and slack condition, allowing free shrinkage. The structural changes due to the heat settings were analyzed by several techniques (differential scanning calorimetry analysis, wide-and small-angle X-ray scattering, and birefringence). The results revealed different recrystallization responses of the fibers to the applied thermal annealings and consequently different shrinkage mechanisms. In the recrystallization of the Nylon 6 fibers were involved a generation of nuclei crystallites in the interfibrillar regions, as well as growth and perfection of these new crystallites and preexisting ones. But, recrystallization of the Nylon 6.6 fiber was accompanied only by growth and perfection of the preexisting crystals. The existence of the nuclei crystallites at temperatures of heat treatments above 120ЊC was the major commanding factor for the Nylon 6 fiber to undergo less shrinkage than the Nylon 6.6 fiber. These very tiny crystallites worked as crosslinking points that would impose restrictions in the mobility of the chains segments, inhibiting subsequent disorientation of the amorphous regions and consequently intense shrinkage, thus resulting in recrystallization in a preferred direction of the fiber axis. The Nylon 6.6 fiber experienced an instantaneous shrinkage at the annealing temperature around 70ЊC. That was the temperature necessary to release its hydrogen bonds and the starting temperature for presenting its major structural changes, including global disorientation of the amorphous and crystalline regions. Thus, its recrystallization occurred with no preferred orientation. Also, it was suggested that the occurrence of so different shrinkage mechanisms reside in the different crystalline morphology that these fibers originally possessed before the heat treatments. ᭧ 1998