The polyphenol and flavonoid contents and antioxidation capabilities, including total reducing power, DPPH radical scavenging capability and lipid peroxidation inhibition, of ethanol extracts of water hyacinth (Eichhomia crassipes) were investigated and compared with those extracted from red tide seaweed (Enteromorpha prolifera) and from the most common seaweed (Porphyra haitanensis). Results showed that E. crassipes contained higher amounts of polyphenols [6.68 mg gallic acid equivalents (GAE)/g] in the leaf than in the stem (4.37 mg GAE/g) or in P. haitanensis (4.27 mg GAE/g). The E. crassipes also contained higher contents of flavonoids [1524 and 453 mg rutin equivalents (RE)/g in leaf and stem, respectively] than did E. prolifera (233 mg RE/g). Accordingly, both the leaf and stem of E. crassipes had relatively higher reducing power [118.35 and 47.21 100μg ascorbic acid equivalents (AscAE)/g, respectively] and DPPH radical scavenging capability (74.6% for leaf and 62.7% for stem) when compared with E. prolifera (reducing power of 16.5 and DPPH radical scavenging capability of 42.96%). Statistical analysis showed that the antioxidation capabilities, especially reducing power, of extracts of the three aquatic plants were positively correlated with their total polyphenol contents, but not with the contents of flavonoids. The correlation coefficient (r) of the content of polyphenols and reducing power of those extracts was r=0.9028, that for DPPH scavenging and lipid peroxidation inhibition was r=0.9311 and 0.9099, respectively. These results indicated that phenolic compounds are the main compounds contributing to the antioxidation activities of seaweed extracts. E. crassipes can be a source for extracting natural antioxidants.