The evolution of the mechanical properties and structure of poly(p-phenylene terephthalamide) (PPTA) fibers with different post-treatment methods involving heat, tension, hydrostatic pressure, and different environments was systematically investigated. Wide-angle X-ray diffraction measurements reveal that the crystal structure of PPTA fiber is not stable and changes upon post-treatment. The cooperative changes in the modulus and two structure parameters - the misorientation angle and the paracrystalline parameter upon treatment indicate a direct structure-property correlation. After studying free-length annealing and heat-tensioning of fibers, several structure parameters - the c-dimension of lattice constants, the paracrystalline parameter, the intensity ratio between(110) and (200), and the orientation angle - were found to be affected greatly by the tension applied during heat stretching; while other structure parameters such as apparent crystal sizes, equatorial crystallinity and a, b dimensions of the lattice constant are insensitive to the applied tension but determined by the applied temperature and time. A sudden change in the crystal structure at 400 degreesC suggests a alpha -relaxation in the crystalline region, which is supported by the DMTR and TMA measurements.