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When silk fiber derived from Bombyx mori was subjected to degumming treatments twice in water and subsequent degraded processing in slightly alkaline aqueous solution under high-temperature and high-pressure, the water-soluble silk sericin peptides (SS) with different molecular mass from 10 to 70 kDa were obtained. The sericin peptides could be conjugated covalently with insulin alone with cross-linking reagent glutaraldehyde. The physicochemical properties of the silk sericin–insulin (SS–Ins) conjugates were determined by Enzyme-Linked Immunosorbent Assay (ELISA). The biological activities of SS–Ins bioconjugates were investigated in vitro and in vivo. The results in human serum in vitro indicated that the half-life of the synthesized SS–Ins derivatives was 2.3 and 2.7 times more than that of bovine serum albumin–insulin (BSA–Ins) conjugates and intact insulin, respectively. The pharmacological activity of SS–Ins bioconjugates lengthened to 21 h in mice in vivo, which was over 4 times longer than that of the native insulin. The immunogenicity of silk sericin and the antigenicity of SS–Ins derivatives were not observed in both rabbits and mice. The bioconjugation of insulin with silk sericin protein evidently improved both physicochemical and biological stability of the polypeptide.