Fucoidans were extracted and isolated by hot-water extraction and anion-exchange chromatography from six species of brown seaweeds were collected in Nha Trang Bay Sargassum polycystum, Sargassum oligocystum, Sargassum denticapum, Sargassum swartzii, Sargassum meclurei and Turbina ornata. The structural characteristics and bioactivities of fucoidans were studied. All fucoidans are sulfated galactofucans, the main components were fucose (32.4-55.9 percent), galactose (14.8-38.7 percent), sulfate (20.40-25.69 percent), uronic acid (7.50-23.74 percent) with a minor amount of xylose, mannose and glucose. The bioassay test results were showed that fucoidans from Sargassum polycystum, Sargassum oligocystum, Sargassum denticapum, Sargassum swartzii, Sargassum meclurei and Turbina ornata demonstrated bioactivity effects against cancer cells.