In the present study, eight species of marine cyanobacteria species namely, Lyngbya confervoides, Nostoc commune, Oscillatoria fremyii, O. geminata, O. sancta, Phormidium corium, P. tenue and Spirulina major were evaluated for their antioxidant activities by in vitro assays. The cyanobacterium, Oscillatoria fremyii (209.49 μmol AAE/g extract) exhibited high total antioxidant activity. Low value was found in Phormidium corium (45.20 μmol AAE/g extract). The radical scavenging capacity, reducing power capacity and chelating activity of the culture extracts varied from 2.29 to 8.62 μmol GAE/g extract, 6.21 to 44.67 μmol GAE/g extract and 2.15 to 30.29 μmol EDTAE/g extract, respectively. The study illustrates that methanol extracts of cyanobcateria isolates have effective antioxidant activities and could be used as natural sources of antioxidants.