Alginate of an average molecular weight(M_w) of approx. 900kDa with the ratio of M(mannuronate)/G(guluronate)about 1.3 in aqueous solution was irradiated with ⁶⁰Co gamma-rays at doses up to 800kGy. With the increase of the irradiation dose Mw of irradiated products was reduced and the dose was correlated with the molecular weight distribution of degraded products. The new bands appeared at 265 nm and 1709cm^-1 in UV and FTIR spectra of irradiated alginate suggested that the scission was accompanied by side reactions leading to the formation of carbonyl groups. The chemical analysis proved the increase of carboxyl groups and reduction of reducing groups of irradiated alginate. The NMR spectra indicated not only the formation of carbonyl groups but also the generation of unsaturated product with C₁ and C₄ double bonds at nonreducing terminal and reducing end product. The degradation mechanism of alginate by irradiation was proposed.