The algicidal activity of the terrestrial plants is a novel and promising means for alga control of eutrophic waters. In this study, three compositae plants -Artemisia annua (A. annua), Conyza canadensis (C. canadensis), and Erigeron annuus (E. annuus) were selected and antialgal activity of petroleum ether (PE), ethyl acetate (EA) and ethanol extracts of these plants on blue algae Microcystis aeruginosa was investigated. The result shows the EA extracts of these plants present the stronger inhibitory effects. The results of the further separation of PE and EA extracts according to water-solubility and acidity or alkalinity show that fatty acids and terpenoids in A. annua, terpenoids and organic acid in C. Canadensis and E. Annuus may be the main antialgal active constituents. This study suggested that the EA extracts of these plants could become the potential algicide to prevent water blooming. compositae plants, fractional extraction, microcystis aeruginosa, antialgal activity