Electron-beam irradiation has demonstrated the promising results for tuning the exothermic behavior of polyacrylonitrile (PAN) fibers, and the stabilization time can be shortened accordingly. However, the mechanical properties, especially the tensile strength, of carbon fibers based on irradiated PAN fibers are usually lower than those of standard carbon fibers. In this study, PAN fibers irradiated with an electron beam to 1200 kGy are continuously stabilized and carbonized to produce carbon fibers (i-CFs). The mechanical properties of the i-CFs are optimized by controlling the density of the stabilized fibers, and the best performing i-CF achieves a tensile strength of 2.27 GPa and a modulus of 174 GPa. The structure and properties of the carbon fibers and the precursor fibers are investigated to determine the origin of the low tensile properties of the i-CFs. The results show that the graphite crystalline structures are comparable between the CFs and the i-CFs. However, two distinct peaks at 2020 and 1668 cm(-1) emerge in the FT-IR spectra of PAN fibers after irradiation that correspond to the C=C=N (ketenimine) and C=N-N=C conjugation groups. These cross-linking structures may disrupt the cyclization reaction and hinder the formation of the aromatic structure of PAN during stabilization, which bring about defects in the resultant carbon fibers.