Herbicidal activities of 29 plant extracts were tested against Pistia stratiotes L. in laboratory by water culture method at 20-25 ℃ and RH 50%-60%. Results showed that 10 of them had obvious herbicidal activities by the indicators of leaf mortality and whole plant mortality and with reference to root response. They were Aralia armata (Wall.) Seem.,Mahonia bealei (Fort.) Carr., Hibiscus syriacus L., Coptis chinensis Franch, Rhus javanica Mill., Datura metel L., Aleurites fordii Hemsl., Dipsacus chinensis Batal, Zanthoxylum simulans Hance and Tripterygium wilfordii Hook. f..Among these 10 plants,Aralia armata(Wall.) Seem. was the best one. When its methanol extracts reached the concentration of 0.625 mg/mL, the corrected leaf mortality of Pistia stratiotes L. was 84.56% and the corrected whole plant mortality was 80.64%. Even at the low concentration of 0.313 mg/mL, the corrected leaf mortality of Pistia stratiotes L. was as high as 55.65% and the corrected whole plant mortality was 38.97%.