The effect of gamma irradiation on the storage stability of Undaria pinnatifida was investigated. A three-log reduction in total aerobic bacteria in U. pinnatifida samples was obtained by irradiation with 3 kGy. During storage for 2 weeks, the viable counts of microorganisms in non-irradiated U. pinnatifida increased to 6 log CFU/g. However, irradiation of U. pinnatifida by 5 kGy eliminated microorganisms. The Hunter of U. pinnatifida was slightly increased after 5 kGy of irradiation. The hardness properties of U. pinnatifida decreased during storage, but were not changed by irradiation with up to 3 kGy. These results indicate that low-dose irradiation (3 kGy) could be effective for improvement of the storage stability of U. pinnatifida.