Abstract: Using the assay systems of DPPH, hydroxyl radical, oxidation of linolic acid induced by alkane radical, antioxidant activity of high-weight phlorotannins from S.kjellma-nianum was studied.The results showed that hydroxyls in phlorotannins are hydrogen donors and the vicinal trihydroxyl is a more active hydrogen donor than meta-trihydroxyl.The hydroxyl radical scavenging capacity is closely related to the number of sites in phlorotannins' benzene rings for the addition of hydrogen radicals.Phlorotannins can inhibit linolic acid oxidation induced by alkane radicals and cannot scavenge superoxide radicals.