The results of X-ray diffraction on polyacrylonitrile-based carbon fibers graphitized at temperatures from 1800 to 2500 A degrees C are reported. The carbon crystallites consist of graphene-layer stacks. A Hirsch analysis showed that the average graphene stack height increased with temperature up to similar to 2000 A degrees C, reaching a relatively stable stack height thereafter. The average lateral layer size increased from 0.88 nm for the as-received fibers to 3.37 nm at 2500 A degrees C. With increasing temperature the formation of microfibrillar structures was observed, accompanied by the flattening of carbon layers. The changes in electrical and thermal properties were in good agreement with the aspect ratio of carbon layers.