PAN-based carbon fiber is a type of brittle material whose fracture surface has a structure peculiar to brittle materials.The tensile strength of a carbon fiber is determined by the strength of weak links in the fiber to a large extent.Since fracture surface is a type of weak link,the fracture surfaces of PAN-based carbon fibers were investigated in this article.SEM(Scanning Electron Microscopy)micrographs of selected carbon fibers with different tensile strengths were analyzed on computer and the fractal dimen-sions of fracture surfaces were obtained on the basis of fractal theories.The relationship between the tensile strength of PAN-based carbon fiber and the fractal dimension of its fracture surface was demonstrated.