Abstract: The hot-water extract of seaweed Sargassum hemiphyllum was evaluated antioxidant activity by using four different methods, including DPPH free radicals scavenging activity, Fe chelating activity, superoxide anion radical scavenging activity and reducing power, and the immune-stimulating activity by using HB4C5 cells and J774.1 cells. It was found that the antioxidant activity was increasing in correlating with the concentration below 3.5 mg/ml. The HB4C5 cells showed the maximum relative activities of cell proliferation (174%) and IgM secretion (132%) at 120 μg/ml of hot-water extract. Overheating slightly reduced the immune-stimulating activity of hot-water extract. At 80 μg/ml, J774.1 cells showed the maximum relative activities of cell proliferation (141%) and phagocytosis (147%). Hence, the hot-water extract of seaweed S. hemiphyllum plays the important role on antioxidant and immune-stimulating activities.