The thermal properties of Bombyx mori silk fibers subjected to heat treatment were examined by thermogravimetry, differential thermal analysis, Fourier transform infrared absorption spectrometry (TG-DTA-FTIR) and scanning electron microscopy (SEM). The color, size, and shape of B. mori cocoon shells were observed as they were heated from 25 to 550degreesC. Only 1% of the original cocoon shell weight remains as cocoon ash after treatment at 550degreesC. Inorganic components, such as calcium, potassium, sulfur, magnesium, etc., were detected in the cocoon ash by energy dispersion fluorescent X-ray spectrometry. A sharp decrease in was observed in the TG data beginning around 280degreesC, and in endothermic peak appeared at 308degreesC, as evidenced by the DTA curves. The IR bands observed at 2380 cm(-1) (-OH stretching), 1760 cm(-1) (C=O stretching), 1503 (N-H stretching), 1085 cm(-1) (C-N stretching) and 965 cm(-1) (-NH2 stretching) become stronger as an exothermic reaction beginning at 280degreesC takes place. This is probable due to cleavage of the main chain and the accompanying decomposition of the silk fibers. Similarly, a SEM micrograph of silk fibers treated at 300degreesC shows a microtubule in the middle of the silk fibers of about 25-mum diameter. This suggests that the thermal reactions starts in the middle of the silk fiber and forms a microtubule.