Fucoidans are sulfated polysaccharides derived from marine brown seaweed. They were reported to exhibit a wide range of physiological and biological activities such as anti-inflammatory, antiviral, anticoagulant, antitumor and antioxidant. Among alga diversity, the brown seaweed Sargassum genus is the largest natural seaweed resource of Vietnam, including about 50 species. Sargassum genus has considerable potential for use in remediation schemes and is enriched in polysaccharides. In this paper, fucoidan was extracted from brown seaweed Sargassum henslowianum collected at Hai Van - Son Cha peninsula, Hue, a central province of Vietnam. Antioxidant and antitumour activities of the fucoidan were investigated. The results indicated that Sargassum henslowianum is a natural resource of some bio-polymer and fucoidan from the seaweed showed great biological activities.