RADIATION induced degradation of chitosan, Na-alginate and carrageenan was carried out to prepare oligosaccharides with different molecular wt. Structural and average molecular wt changes of such oligosaccharides were determined by gel permeation chromatography (GPC), Fourier transform infra-red (FT-IR) and ultraviolet (UV-Vis.) spectroscopy. FT-IR and UV-Vis. studies revealed that during radiation degradation process, the main polysaccharide chain structure was almost remained. Comparative study on the antioxidative properties of chitosan, Na-alginate and carrageenan oligosaccharides of different molecular wt was investigated. Radical mediated lipid peroxidation inhibition, scavenging effect on 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals, reducing power and the ferrous ion chelating activity assays were used to evaluate the antioxidant activity. The irradiation process enhances the antioxidant activity of such polysaccharides. The lower the molecular wt of oligosaccharides is the higher the antioxidant activity. The antioxidant activity of irradiated chitosan was higher than Na-alginate and carrageenan. At 30 kGy, the inhibition concentration (IC50) on DPPH radicals was 0.154, 0.359, 0.438 and 0.140 mg/ml for chitosan, Na-alginate carrageenan, and ascorbic acid, respectively.