Abstract: Carbon fibers prepared from boron-doped polyacrylonitrile gel fibers were graphitized at 1600~2400℃, and the graphitized fibers were characterized by inductively coupled plasma atomic emission spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction and mechanical tests to investigate the influence of boron on their microstructures and properties. The results showed that the boron content in the carbon fibers decreased with increasing heat treatment temperature. Boron occupied substitutional sites in carbon network and moved by diffusion during graphitization. The development of crystallite size was not affected but the orientation of the fibers was significantly improved by the low boron content in the fibers, which made the Young's modulus increase substantially and the loss of tensile strength was reduced. The graphitizing temperature of the boron-doped carbon fibers was lowered by 200℃ in order to prepare samples with the same Young's modulus.