Abstract: Si-B co-doped pitch-based carbon fibers were prepared from mixtures of polyborosilicane (PSNB) and petroleum pitch with a PSNB content of 3 to 10 wt% to improve their oxidation resistance. The composition, microstructure, mechanical properties and oxidation resistance of the fibers were investigated. Results show that with increasing PSNB content in the pitch, the tensile strength and Young's modulus decrease gradually, but the oxidation resistance increases significantly. For fibers prepared by carbonization at 1400℃, the weight loss is 25% after oxidation at 600℃ for 240 min and 60% after oxidation at 650℃ for 140 min. However, these values are 46% and 99% of the respective values for the un-doped carbon fibers. The B₂O₃ generated during oxidation of the Si-B co-doped carbon fibers has a good fluidity, and as a result a continuous glass film is formed on the surface of the fibers, which effectively inhibits carbon oxidation.