A commercial method for permanent hair straightening process including three chemical reaction steps: i.e., reduction, heat treatment and subsequent oxidation has been widely used. The effects of reduction treatment on chemical, morphological and fine structural changes of hair were studied. Reduction was performed by using a thioglycolic acid (TGA) only and a bicomponent system composed of TGA and dithiodiglycolic acid (DTDG). Our approach was focussed on the effects of DTDG on hair damage. Amino acid analysis showed that in the presence of DTDG, the reduction of disulfide bonds is suppressed, while the formation of thiol groups from the mixed disulfide which is made by the reaction of cystine thiol and thioglycolic acid is enhanced. The results of high pressure DSC and the histochemical observation of cortical cells suggested that the hair straightening treatments give rise to a destruction of the α-crystallites in part and more or less degradation of either the non-keratinous components or the amorphous matrix. However, FT-IR spectral data suggested that the α-helical materials are present in the straightened hairs at the same level as detected in the untreated hair. The reducing system containing DTDG decreased chemical attack on the amorphous matrix and the structural regularity of the matrix remained to be almost unchanged.