The transformation of the internal strain and its effect on the microstructure of polyacrylonitrile-based carbon fiber during the high-temperature graphitization were investigated.The internal compressive strain within the carbon turbostratic structure was confirmed through a careful analysis by wide-angle X-ray diffraction and Raman spectroscopy.Heat-induced strain/stress relaxation along the fiber axis was observed and was found to have a profound effect on the structure of both the crystallites and microvoids.The results indicated that,the relaxation of residual strain changed the graphite layers from a wrinkled and distorted morphology to a straight and smooth one,and consequently led the crystallites to stack closely and orderly with increasing stack height.The strain relaxation also changed the morphology of crystallites and microvoids,resulting in an anisotropic growth for the latters. The transformation of the internal strain and its effect on the microstructure of polyacrylonitrile-based carbon fiber during the high-temperature graphitization were investigated.The internal compressive strain within the carbon turbostratic structure was confirmed through a careful analysis by wide-angle X-ray diffraction and Raman spectroscopy. Heat-induced strain / stress relaxation along the fiber axis was observed and was found to have a profound effect on the structure of both the crystallites and microvoids. The results indicated that, the relaxation of residual strain changed the graphite layers from a wrinkled and distorted morphology to a straight and smooth one, and led led the crystallites to stack closely and orderly with increasing stack height. strain the strains also changed the morphology of crystallites and microvoids, resulting in an anisotropic growth for the latters.