Kevlar 49 fibres were annealed in the temperature range(150-500)°C in nitrogen gas atmosphere, or thermally aged at (150°C) in static air. Their fine structure was determined in terms of  the crystallinity and microparacrystallite  lateral sizes using wide-angle X-ray diffraction techniques. Their mechanical properties were measured in terms of initial Young's modulus,tensile strength and elongation at break. Relationships between the crystalline and mechanical properties have been pointed out. A significant results in the empirical equation relating to the initial Young's modulus(M) and the crystallinity (C) , M = 3.27C – 133, which leads to a modulus of (194GPa) for a (100%) crystalline Kevlar 49 , supporting the theoretical modulus values (180-200GPa)  for poly (p-phenylene terephthalamide (PPTA) fibres and modulus value (218GPa) for Kevlar 49 fibres if they had no paracrytalline lattice distortion.