The present work compared the recrystallization process of Nylon 6 with Nylon 6.6 fibers. For such a study, the fibers were submitted to different annealing conditions (slack and restrained conditions) in a wide range of temperatures. For the structural analysis, several techniques were applied, and among them, differential scanning calorimetry (DSC) was demonstrated to be a very important tool. Nylon 6 and 6.6 fibers responded differently to the applied annealings, indicating different recrystallization mechanisms. The Nylon 6 fiber presented the formation of new and very small crystallites in their interfibrillar regions for the annealings performed above 120 degrees C independently of the annealing condition. In addition to their improvement in size and perfection as the annealing temperature increases, their presence favored a general recrystallization in a preferred direction, that is, of the fiber axis. The recrystallization process of the Nylon 6.6 was commanded by the disorientation process associated to the release of the hydrogen bonds. The DSC thermograms revealed two crystalline forms, that is, of the same type but with different degrees of size and perfection. At temperatures below the T-g, the less perfect crystallites are converted into more perfect ones, while at temperatures above the T-g, the intense movement of the chain segments favored a reversal in this process, that is, the more perfect ones are converted again to the less perfect form.