Fucoidan is a group of fucose-containing sulfated polysaccharides mainly derived from brown seaweeds,crude extracts of which are commercially available as nutritional supplements.Fucoidan has a backbone built of (1→3),(1→4),and (1→2)-linked α-L-fucose residues and also include sulfated with fucose or fuco-oligosaccharide branching,or other monosaccharide substitutions.Recent studies have demonstrated a variety of beneficial biological functions of fucoidan,including anticoagulant,antitumor,antithrombosis,antivirus,antioxidation as well as immunbomodulation.The content of fucoidan in brown seaweeds is not high,but its prominent physiological activities are attracting attentions from many industries all over the world.The versatile bioactivities vary depending on the source of seaweed,the structural and compositional properties,the sulfate content and substitution position,etc.The structural integrity of fucoidan molecular essentially due to the extraction methods is different,which are crucial for obtaining and elucidating structure-bioactivities relationship.This article discusses the fucoidans preparation,structure-bioactivity relationship from brown seaweeds.