The evidences demonstrate that free radicals, especially superoxide and hydroxyl, have an important role in mediating the inflammatory process. These properties have taken some researches to evaluate the antioxidant capacity from various compounds. Considering the limited research with polysaccharides and knowing its largely employed by the pharmaceutical and foodstuffs industries, our have objective in to verify its action as antioxidants. Six sulfated polysaccharides were investigated in their in vitro antioxidant activities: iota, kappa and lambda carrageenans, widely used in the food industry, fucoidan (homofucan) from edible seaweed Fucus vesiculosus and fucans (heterofucans) F0.5 and F1.1 from seaweed Padina gymnospora. The results obtained for inhibition of superoxide radical formation showed the fucoidan with IC50 of 0.058 mg/mL while IC50 for the kappa, iota and lambda carrageenans were 0.112, 0.332 and 0.046, mg/mL respectively. The results obtained for inhibition of hydroxyl radical formation demonstrated that all the sample had effect on the inhibition of the formation of these radicals. The peroxidation tests showed an IC50 of 1.250 mg/mL for fucoidan, 2.753 and 2.338 and 0.323 mg/mL to kappa, iota and lambda carrageenans, respectively. The fraction of fucans showed low antioxidant activity when compared to fucoidan.These results clearly indicate the beneficial effect of algal polysaccharides as antioxidant.