The microstructural transformation of wool fiber during the stretching process was investigated by the tensile tests and TEM micrograph observation. The results showed that water breaks some of the hydrogen bonds in the fiber, whereas the sodium bisulfite solution breaks some of the disulphide crosslinks between the peptide molecules, then the wool fiber is easy to be stretched. Two types of transformations were found during the stretching process, that is, the slippage of molecule chains or micro-structural elements and the a p transformation. The a-P transformation is dominant, in particular, during the low stretching rate process. In addition, the stretching slenderization tends to increase the tenacity of wool fiber and decrease its extension at break. (c) 2007 Wiley Periodicals, Inc.