Abstract: To discuss the difference in antioxidant activity among different algal polysaccharides, we analyzed the free radical-scavenging and anti-lipid peroxidation capacity of crude polysaccharides extracted from Betaphycus gelatinae, Ulva lactuca, Sargassum polycystum, Padina australis and Caulerpa sertularioides in vitro. The results indicate that five algal polysaccharides were significantly different in antioxidant activity. The polysaccharides of S.polycystum and P.australis showed strong reducing power and free radical-scavenging capacity for superoxide anion and hydroxyl radical. The scavenging capacity for superoxide anion of P.australis polysaccharide IC₅₀=(262.00±24.60) μg·mL^-1 was significantly higher than that of S.polycystum IC₅₀=(458.00±18.70) μg·mL^-1, and the scavenging capacity for hydroxyl radical of P.australis polysaccharide IC₅₀=(388.00±45.29) μg·mL^-1 was similar with that of S.polycystum IC₅₀=(312.04±37.42) μg·mL^-1.The scavenging capacity for DPPH of S.polycystum polysaccharide IC₅₀=(95.80±7.48) μg·mL^-1 was significantly higher than the other algal polysaccharides, while the scavenging ability for DPPH of P.australis polysaccharide was not good enough IC₅₀=(726.00±54.90) μg·mL^-1.According to the anti-lipid peroxidation analysis in vitro, P.australis polysaccharide had higher inhibitory capacity for liver cell membrane lipid peroxidation IC₅₀=(283.67±44.14) μg·mL^-1, and certain ability to protect red cell lysis which was induced by hydrogen peroxide IC₅₀=(335.50±22.47) μg·mL^-1.