Abstract The growth mechanism for the graphite crystallites in polyacrylonitrile (PAN)-based carbon fibers, heat-treated at various temperatures, has been proposed. The evolution of distribution morphologies for graphite crystallites is investigated, in relation to variations in the tensile properties of the fiber samples. At high temperatures, dangling bonds are created via the cleavage of weak bonds, such as C N, C H and C C at the edges of the graphite crystallites. The graphite crystallites grow through the bonding reactions between different dangling or dangling bonds and agraphitic carbon atoms. This results in graphite crystallites that increase in size with elevated temperatures that lead to changes in graphite crystallite distribution morphology. Between 1400 and 2400 °C, the distribution morphologies of the graphite crystallites go though three states: (I) a dispersed state, (II) a network state, and (III) a transfixion state. The tensile strength decreased rapidly with increasing heat treatment temperature for fibers with a dispersed state structure, but lowers with fibers bearing a network state structure. The decrease was more rapid for fibers with a transfixion state structure. The tensile modulus increased slowly in states I and II, and rapidly in state III.