This study was aimed to examine the antioxidative and immune activity of the water extracts from brown seaweed Sargassum hemiphyllum, Undaria pinnatifida and Laminaria japonica. The water extract from S. hemiphyllum was characterized by higher percentages of total sugar and sulphate, and low molecular weights (<504 Da as major). The antioxidative activity of three water extracts was evaluated using four different methods, including reducing power, DPPH free radicals scavenging activity, Fe+2 chelating activity and SOD-like activity, and the immune activity was evaluated using HB4C5 and J774.1 cells. It was found that antioxidative activity of S. hemiphyllum water extract was higher than U. pinnatifida and L. japonica. In immune activity, HB4C5 cells showed the maximum cell proliferation (135%, 115% and 129%) and IgM secretion (133%, 129% and 126%) at 500 μg/ml of water extracts from S. hemiphyllum, U. pinnatifida and L. japonica., respectively. At 250 μg/ml, J774.1 cells showed the maximum cell proliferation (142%) and phagocytosis (138%) of water extract from S. hemiphyllum. And at 500 μg/ml, J774.1 cells showed the maximum of cell proliferation (119% and 121%) and phagocytosis (112% and 118%) of water extracts from U. pinnatifida and L. japonica. The results suggested that water extract from S. hemiphyllum may be used as a new substance for immunomodulatory.