A sericin hydrogel membrane with a good dynamic property was prepared by chemical cross-linking, and the relationship between the structure and the physical properties of the membrane was studied. The chemical cross-link increases with increasing dimethylolurea (DMU) as a cross-linking agent even after the extraction of the excess DMU. The increase in the cross-link strains the segmental motion, but that motion becomes easier as a mole ratio of 0.5 increases because of the extraction of the excess DMU and the turbulence of the crystal structure of sericin. Water content is lowered to a 0.25 mole ratio of DMU, and then increases to over 0.5 again. Young's modulus shows a tendency that is reversing the result for water content.