The linear low-density polyethylene (LLDPE) fibers were sulfonated in 95 % concentrated sulfuric acid. The structure composition and thermal stability of the polyethylene (PE) fibers sulfonated for various times were determined by Fourier transform infrared spectroscopy (FTIR), resonance Raman spectra, elemental analysis, X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA). It was confirmed that substitution, oxidation and desulfonation took place in succession during sulfonation. The initial increase of sulfonic acid and hydroxyl groups, as an indication of the substitution reaction, led to a proposal that the reaction was initiated by the abstraction of hydrogen by H2SO4 to give PE radical, which reacted with H2SO4 to give HSO3 center dot and HO center dot radical, and terminated by the formation of PE-SO3H and PE-OH. The subsequent decrease of hydroxyl groups and generation of carbonyl groups were attributed to the oxidation reaction. The formation of C=C double bonds and the decrease of sulfur and sulfonic acid groups later on implied that the desulfonation reaction proceeded to induce intermolecular cross-linking. The cross-linked and conjugated polyenes derived from the desulfonation probably attributed to the high char yield for sulfonated LLDPE fibers.