PET fibers of different drawn ratios were used as substrates to produce composites with PANi. The influence of the growth of the polyaniline molecules in the structure of the fibers previously pretreated in the presence of aniline at different conditions were studied by different characterization techniques (differential scanning calorimetry, X-ray scattering, dynamic and mechanical thermal analysis, sonic modulus, and scanning electron microscopy), The polymerization of the aniline inside the fiber substrates modified the structure of the fibers. In the case of the drawn fiber substrates the growth of the PANi molecules occurred mainly in the interfibrillar regions, which promoted reorganization of the preexisting crystals such as defect elimination (mainly in the interface of these regions with the lamellar regions) and at same time reorientation of their amorphous and crystalline regions. Also, due to this reorientation process the crystals became not only bigger and more perfect, but also they became more homogeneous. In the case of the undrawn fiber substrates, due to the lack of previous orientation, the growth of the PANi molecules would be occurring in the amorphous interlamellar regions only. Finally, the use of fibers with different drawn ratios as substrates for the composites was essential to reach such conclusions.