Objective To investigate the antioxidant activity of Enteromorpha prolifera by ABTS radical scavenging assay in vitro and provide scientific basis for exploitation of Enteromorpha prolifera.Methods The antioxidant activities of extracts by different solvents from Enteromorpha prolifera and water extracts from different Enteromorpha prolifera samples were investigated by ABTS radical scavenging assay.Moreover,the antioxidant ability of the water extract from Enteromorpha prolifera was evaluated by using caffeic acid and ascorbic acid as positive control standards.Results The water extract from Enteromorpha prolifera provided the highest radical scavenging activity,and the antioxidant capability of 1mL of water extract from Enteromorpha prolifera was equivalent to 1mL of 2.55 10~(-3) mg?mL~(-1) caffeic acid or 1mL of 2.89?10~(-3)mg?mL~(-1) ascorbic acid.However,the extraction methods,dry sample or not and different kinds of Enteromorpha prolifera samples had certain influence on the antioxidant activity of water extract from Enteromorpha prolifera. Conclusion There are abundant antioxidant compounds present in the aqueous extract of Enteromorpha prolifera,and it can be exploited as latent oxidation inhibitor.