The crystallization and the self-assembly of blending system, syndiotactic polystyrene/polystyrene-block-poly(ethylenepropylene) (sPS/PS-PEP), were investigated by transmission electron microscopy (TEM), polarized light microscopy, small angle X-ray scattering and differential scanning calorimetry (DSC). Spherical microdomains with sPS embedded in PS-PEP matrix were obtained after melt mixing as evidenced by TEM observations combined with DSC analyses. The size of spherical microdomains is in the order of tens nanometer. This unique morphology provides an appropriate system to examine the effect of crystallization on microphase-separated morphology. The driving force of sPS crystallization leads the growth of sPS crystals to overcome the effect of spatial confinement and the repulsive barrier of immiscibility, and thus to go across the surrounding PEP domains. As a result, the growth of crystallization interconnects sPS microdomains and forms crystalline lamellae. The overall crystallization rate of sPS in self-assembly sPS/PS-PEP blends increases with increasing the content of sPS. We suggest that the increase on the crystallization rate is attributed to the decrease on the crossing-distance between the crystallizing sPS domains due to an increase on the size of spherical microdomain. (C) 2003 Elsevier Science Ltd. All rights reserved.