Background and Aim: Marine algae are one of the richest sources of natural antioxidants. In this study, the antioxidant properties of the extract of seaweed Rhizoclonium riparium investigated.
Methods: The algae were collected from the coasts of the Oman Sea, and washed with sea water and tap water. The algae were then dried and powdered with an electric mill. Methanol, chloroform, hexane and dichloromethane used as solvents (1:4). The antioxidant properties of the extracts were evaluated using three methods of free radical scavenging (DPPH), metal ion chelating and reduction power.  
Results: the chloroform extract of algae Rhizoclonium riparium at the concentration of 1 mg/ml showed the most antioxidant properties with free radical scavenging activity DPPH, 94.8±4.63%; chelating activity, 30.8±0.01%; and reduction power, 0.7. The lowest IC₅₀ recorded in Choloroform extract.
Conclusion: the chloroform extracts of algae Rhizoclonium riparium at the concentration of 1 mg/ml had the highest antioxidant activity and this extract is recommended for in vivo studies and identification of its active antioxidant compounds.