Relaxation of shear-enhanced crystallization in an impact-resistant polypropylene copolymer (IPC) upon thermal treatment was systematically investigated, from the sight of phase structure and resultant morphological evolution. Shearing was capable of accelerating the crystallization of IPC, while upon melt annealing, a spontaneous morphological evolution can be observed accompanied by a gradual relaxation of shear-enhanced crystallization. It was found the underlying structural motion during relaxation is in essence a phase separation process towards a multi-layered phase structure, in which crystallizable polyethylene segments are spatially entangled with ethylene-propylene random copolymer chains. The shear-enhanced crystallization originates from the destruction of this multi-layered phase structure which releases crystallizable PE segments to contact the matrix serving as nuclei for the crystallization of polypropylene. Upon annealing, reconstruction of the multi-layered structure can be achieved leading to the relaxation of shear-enhanced crystallization. A conceptual model describing this relationship between phase structure and crystallization behavior in IPC has been proposed. (C) 2010 Elsevier Ltd. All rights reserved.