This study was performed to quantitatively and qualitatively estimate the effect of keratin essence on hair protection against physicochemical damage. Damaged hairs were obtained from an early thirty woman who dyed her hair two times and did digital permanent treatment of her hair two times. The damaged hairs were divided into four experimental groups, which are the control hair (CH) group without additional beauty treatment, the damaged hair (DH) group by additional dyeing treatment, basic essence-treated hair (BEH) group, and keratin essence-treated hair (KEH) groups according to the research goal. The protection effect of keratin essence against the physicochemical damage was quantitatively compared by difference of chrominance measured using a color difference meter and qualitatively compared by difference of outer morphological structure images pictured using scanning electron microscopy (SEM). The brightness and yellowish blue color of KEH were relatively lower but the reddish blue color was relatively higher than other groups of test hairs. Cuticle structure of the previously DH was irregularly deformed and more strongly deformed or partially broken by additional dyeing treatment. On the other hand, the gaps between cuticle scales of the DH were reformed by treatment with basic essence and reformed and filled by treatment with keratin essence in comparison with the DH group. Conclusively, the keratin essence was effective to protect hair structure against the structural damage induced by the dyeing-treatment, by which the coloring efficiency is thought to be improved.