The influence of hot stretching graphitization on the structure and mechanical properties of rayon-based carbon fibers was studied. It was observed that the Young's modulus of the treated fibers increased with heat treatment temperature (HTT) and hot stretching stress, to 173 GPa by 158.2 % through hot stretching at 2700 degrees C under stress of 270 MPa compared to that of the as-received carbon fiber. Meanwhile the tensile strength increased to 1.75 GPa by 73.3 % through hot stretching at 2700 degrees C under 252 MPa. The field emission scanning electron images showed markedly increased roughness on the external surface and bigger and more compacted granular morphologies on the cross section of the treated fibers with increasing HTT. The preferred orientation of graphitic layers was improved by hot stretching, and the higher the HTT, the stronger the effectiveness of the hot stretching. The crystallite sizes grew and the crystallite interlayer spacing decreased obviously with increasing HTT but changed just slightly with increasing stretching stress. The analysis based on uniform stress model and shear fracture theory proposed that the improvement of tensile strength and Young's modulus for rayon-based carbon fiber was mainly due to the increased preferred orientation and nearly unchanged shear modulus between planes with increasing HTT during hot stretching graphitization, which was much different from polyacrylonitrile-based carbon fibers.