Semi-interpenetrating polymer networks (SIPNs) composed of silk fibroin (SF) and a poly(ethylene glycol) (PEG) diacrylate macromer were synthesized and characterized. The conformation of SF in SIPNs was changed from a random coil to a p-sheet structure by formation of SIPNs with PEG, while the crystalline structures of SF and PEG in SIPNs was not changed. The decomposition temperature of SF in the SIPNs was inner-shifted, an indication of SIPN formation between the SF and the PEG macromer. The tensile strength and elongation the SIPNs were much higher than were those of SF itself or the SE/PEG blend and increased with an increase of the PEG content. The equilibrium water content of SF was remarkably increased by formation of SIPNs with PEG due to the hydrophilic property of PEG.