Degradation of chitosan in swollen state with hydrogen peroxide solution (1-5 percent w/v) by gamairradiation was investigated. Molecular weight (Mw) of irradiated chitosan samples was determined by gel permeation chromatography (GPC). Fourier transform infrared (FT-IR) spectra and X-ray diffraction (XRD) patterns were utilized to study the structure of degraded chitosan. Results showed that low Mw chitosan of ~30-45 kDa was efficiently prepared by gama-irradiation of chitosan (Mwo ~ 91.7 kDa) swollen in hydrogen peroxide solutions at low dose less than 20 kGy. The main structure as well as the degree of deacetylation of degraded chitosan was almost unchanged. Furthermore, the radiation degradation yield (Gs) )Was remarkably enhanced with the presence of H2O2 as a sensitizer. Degradation of chitosan in swollen state with hydrogen peroxide solution by gama-irradiation is efficient due to synergistic effect of gama-ray/H2O2 at low dose.