Antioxidant activity of polysaccharide from Agaricus bisporus was studied in vitro by measuring its reducing power,its chelating ability on Fe2+ and its scavenging ability on DPPH ,O-2·and·OH.The results show that polysaccharide from Agaricus bisporus posses stronger reducing power and chelating activity,differdnt scavenging activity on DPPH,O-2·and·OH,namely polysaccharide from Agaricus bisporus have good antioxidant activity in vitro.