Abstract: Boron-catalyzed graphitization of carbon fibers was conducted in a boron-doped graphite crucible at 2 000 ℃, in which boron diffused from the crucible to the carbon fibers to act as the catalyst. The microstructures, boron contents and mechanical properties of the resulting carbon fibers were characterized by Raman spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and mechanical tests. Results indicate that boron has a significant impact on the fiber microstructure. Raman spectra indicate the presence of distortions in the graphitic layers. The tensile modulus of boron-doped carbon fibers obviously increases with boron content and the tensile strength is also higher than that of the original carbon fibers at boron concentrations between 0.58 to 0.68 at.%.