Silk peptide used as a co-initiator in type 2 photo-initiator has not been known. In this study, the photoinitiation system is composed of the sensitizer, the synergist and silk peptide with low molecular weight (<3,500), which is used to initiate photopolymerization of common monomer. The same photopolymerization was performed to evaluate the co-initiating ability of four essential amino acids of silk peptide, and of four pentapeptides synthesized using them as substrates, respectively. It was suggested that the glycine residue and alanine residue on silk peptide might be mainly responsible for the co-initiating role. And the co-initiating function might be ascribed to the electron/hydrogen donation of silk peptide. Silk peptide combined with the sensitizer and iodonium salt could also efficiently initiate photo-curing of hydrogel precursors. And due to excellent water solubility of silk peptide, the water absorption of the formed hydrogel could be further improved. These studies will open an opportunity for the application of silk peptide as a biocompatible co-initiator. POLYM. ENG. SCI., 58:2185-2191, 2018. (c) 2018 Society of Plastics Engineers