The crystallization and melting behaviors of weakly segregated diblock copolymer (MPEG-b-PLLA) have been investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). The microphase separation of MPEG-b-PLLA significantly influenced the crystallization behavior of PEG and the melting behavior of PLLA block, which formed during the two-step crystallization process. PLLA block crystallized from melt under various cooling rates (R-c, (PLLA)s) in the first step, and PEG sequentially crystallized in the second process as the temperature cooled from 60 degrees C to -70 degrees C at 10 degrees C/min. With R-c,R- PLLA decreasing, PEG was more excluded out of the interlamellar regions of PLLA crystals, and concentrated into the interfibrillar regions, and formed the different microphase separation formed, which may attribute to the fractional crystallization and homogeneous nucleation. Then the melting peak of PLLA at lower temperature enhanced with the micro-domains of PEG growing, which could be ascribed to that PEG block could accelerate the melting of PLLA near these micro-domains. (C) 2015 Elsevier Ltd. All rights reserved.