Crystallization kinetics and melting behavior of PEEK were studied by differential scanning calriometry (DSC) and modulated differential scanning calriometry (MDSC). The isothermal crystallization was performed in DSC between 290 and 320¢XC. The Avrami constants (n1, n2) and the level off time were determined from the Avrami analysis. The n1 values varied from 1.50 to 2.98, and the n2 values were between 0.52 and 1.37. The minimum induction time required for the occurrence of double melting peaks was obtained by increasing the isothermal crystallization time in a interval per minute. It was found that the minimum time was always longer than the level off time, which cannot be used as the delimitation for the occurrence of single or double melting peaks. To study the melting behavior and the mechanisms of double melting peaks, the samples after melting at 400¢XC for 15 min were crystallized isothermally between 200 and 320¢XC for 10 or 60 min, and then they were heated to 380¢XC at 10 or 2 ¢XC/min, respectively. From the MDSC results of crystallization temperatures between 280 and 310¢XC, it is found that two different morphologies and melting-recrystallization phenomenon coexisted. As the isothermal crystallization temperature increased from 280 to 310¢XC, the contribution of melting-recrystallization to the upper melting peak gradually decreased. In the case of 320¢XC, the mechanisms of double melting peaks were dominated by two different morphologies only. Quasi-isotropic composites in the stacking sequence of [0/¡Ó45/90]2s were fabricated by a modified diaphragm forming apparatus. Three different processing conditions were used to prepare AS4/PEEK composites with the same crystallinity but different transcrystallinity. The morphology before and after the long-term tensile-tensile tests was observed by means of scanning electron microscope. The transcrystallinity has no significant effect on the short-term tensile test. This was due to the fibers in the 0¢X plies of [0/¡Ó45/90]2S laminates dominated the failure at high stress for the short-term tensile test. However, as the transcrystallinity increased, the failure cycles for the long-term tensile test became longer. This expressed that the delay of damage initiation in the 90¢X and ¡Ó45¢X plies of [0/¡Ó45/90]2s led to a longer failure cycles in the long-term tensile tests.