The effects of sodium alginate on the physical properties of fibroin membranes, such as rupture elongation, rupture strength, water absorption, and thermal properties, were investigated. The experimental results showed the beta-form crystal and beta-form molecular conformation increased due to the increase of intermolecular hydrogen bonds, such as C = O---HN and C = O---HO. The kind of intermolecular interaction and beta-form molecular conformation of polymer blends were clarified by C-13-NMR and infrared spectroscopy. The increase in beta-form crystal was examined by X-ray analysis. Further, membranes were characterized by thermogravimetric analysis (TGA) and by determining their water contents and tensile properties.