The scavenging effects of three fucoidan fractions ( LMWF-M, LMWF-I, and LMWF-IV) of low molecular weight extracted from Laminaria japonica on free radicals of active superoxide anio( O -.. 2 ) and hydroxyl ( ..OH) were studied by means of the chemiluminescence analysis. The experimental result s showed that three f ractions all exhibited signif icant scavenging capacity on O -.. 2 which was increased with the increase of LMWF concentration. LMWF- I containing high glucuronic acid but low sulfate content displayed the highest scavenging effect on free radicals of act ive oxygen , and its IC50 on O2 -.. was 0. 044mg..mL - 1 and 0. 062mg..mL - 1 on ..OH. Moreover, LMWF- M showed strong antioxidative activity in the experimental rat with hyperlipidemia. The content of LPO in serum and tissue was greatly reduced in hyperlipidemic rats( P < 0. 01) , while the activity of SOD was effectively enhanced ( P < 0. 01) .