The degradation behavior of hydrogel based on poly(aspartic acid) (PASP) which crosslinked by 1, 6-hexamethylene diamine (HD) was studied in alpha-chymotrypsin solution. The degradation behaviors of PASP under different crosslinking levels were evaluated as a function of biodegradation time by monitoring the changes of crosslinking density, molecular weight, rheological behavior, interior morphology, mass loss, and swelling ratio. It was found that the degradation behaviors relied heavily on the crosslinking density which calculated from the Flory-Rehner equation. A pseudo first-kinetic equation was proposed to model the mass loss behavior and the degradation rate constant was calculated under different crosslinking levels. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 178-185, 2010