In order to investigate the graphitization behavior of Bombyx mori silk fibroin (SF) fibers, structural inspection of carbonized SF fibers treated with iodine vapor was studied at different temperatures, from 800 to 1400 A degrees C. Wide angle X-ray diffraction measurement suggested that both untreated and iodinated SF fibers exhibited amorphous structure carbonized to 1200 A degrees C. After carbonization at 1400 A degrees C for 12 h, a graphite-like structure was obtained. Raman spectroscopy and transmission electron microscopic observation showed that the graphite layers of SF fibers became more ordered after iodine treatment. The carbon yield obtained at higher temperature (1400 A degrees C) after iodine treatment was higher, ca. 28 wt%, than that of untreated SF.