Purpose: To investigate how silk amino acids affect the hair by applying them to bleached hair.Methods: A silk amino acid formulation was prepared by adding 0, 3, 6, and 9 g of silk amino acid content. The prepared formulation was applied to bleached hair. To determine the change in hair brightness, the L* value was measured using a colorimeter. Additionally, the hair’s tensile strength, absorbance using methylene blue, and gloss were measured to determine whether amino acids affect bleached hair. Then we performed a comparison of data between the sample hair with the formulation agent containing silk amino acid and damaged hair.Results: Regarding brightness, the L* value for each sample increased as compared with that of damaged hair. Additionally, the brightness increased as the silk amino acid content increased. Moreover, the tensile strength of all samples with perm-base and silk amino acid added increased as compared with the damaged hair. The tensile strength also increased as the silk amino acid content increased. The absorbance of all samples with perm-base and silk amino acid added decreased as compared with that of damaged hair, and as the silk amino acid content increased, the absorbance also decreased. Regarding gloss, the gloss of the samples with perm-base decreased as compared with that of damaged hair. However, as the silk amino acid content increased, the gloss also increased.Conclusion: There were differences in brightness, absorbance, and gloss between damaged hair and bleached hair with silk amino acid, suggesting that silk amino acids have an effect on damaged hair. In the future, studies investigating on whether various natural ingredients affect damaged hair are warranted.