Kevlar 29 is an aromatic polyamide fiber spun from the polymer poly(p-phenylene terephthalamide). It has a paracrystalline structure with a lattice distortion parameter g = 5.9% and an a* constant = 0.51. The equatorial x-ray “crystallinity” is 68%; the microparacrystal (mPC) sizes areD002= 50 nm,D110= 4.4 nm, andD200= 3.6 nm. The mPCs are well oriented; the orientation parameter <sin2ψ200> = 0.047. The small-angle x-ray diffraction pattern indicated that the fiber has nc chain folding. It has high tenacity (2.81 GNm−2), close to the tenacity of steel; high Young's modulus (63.9 GNm−2); small breaking strain (3.8%); and a very high melting point (600°C). Wide- and small-angle x-ray diffraction techniques, differential scanning calorimetry (DSC), and an Amsler tensile tester have been used to characterize the fiber and to assess the effect of annealing on its morphology and mechanical properties.