Poly(aspartic acid) (PAsp) hydrogels have been prepared by the gamma-irradiation of PAsp produced by thermal polycondensation reactions with an acid catalyst. The effects of the molecular weight of PAsp, pH,, concentration of PAsp in the aqueous solution and dosage of gamma-irradiation on the PAsp hydrogel preparation were investigated. PAsp hydrogels were prepared in the case when the PAsp weight-average molecular weight was 95,000, the pH of the solution was 7.5 or higher, the concentration of the solution was 5-10 wt/vol%, the dosage of gamma-irradiation was 32 kGy or more and the preparation of PAsp solutions was done under Nz. On the other hand, PAsp with low-molecular weight (M-w = 15000) could not form hydrogel by gamma-irradiation. The swelling of PAsp hydrogels in deionized water and artificial urine was measured. The maximum swelling by deionized water was 3400 g-water/g-dry hydrogel. Biodegradation of the hydrogel was checked using the activated sludge (Japanese Industrial Standards (JIS) K6950 method) indicating that the hydrogel showed about 50% biodegradation for 28 days.