Modified poly(aspartic acid)s containing pendant allyl groups were synthesized by the reaction of poly(succinimide) with an allyl amine in dimethylformamide. The contents of the allyl groups in the poly(aspartic acid) ranged from 2 to 17.4% confirmed by (1)H NMR. Hydrogels were prepared using modified poly(aspartic acid) by chemical crosslinking using redox radical initiators including ammonium persulfate and potassium peroxodisulfate. The morphologies of the poly(aspartic acid)-based hydrogels were investigated by scanning electron microscopy (SEM). The water-absorbent experiments were carried out, and revealed that lightly cross-linked hydrogels resulted in effective water-absorbent properties. These results suggested that allyl group-modified poly(aspartic acid)s are useful in providing biodegradable hydrogels.