Abstract: Changes in the structure and mechanical properties during the conversion of lyocell precursors to carbon fibers were investigated by scanning electron microscopy, wide angle X-ray diffraction and small angle Xray scattering when they were treated with a catalyst containing sulfuric acid and carbamide. It was found that the density of fibers first increased with temperature, then decreased when the temperature exceeded 200℃. The strength decreased gradually and was close to zero between 150℃ and 200℃, but then increased with increasing temperature. The degree of polymerization showed similar changes to the strength changes during pyrolysis. The crystalline structure of the cellulose II crystals in lyocell fibers was converted to a 2D turbostratic carbon structure.