The herbicidal activity of the extract of Taiwania flousiana Gaussen was evaluated by the cup method.Results showed that the fraction of ethyl acetate at 100 μg/mL completely inhibited the growth of Ageratum conyzoides L.The inhibition rates of root length and fresh weight were 100%.Two active ingredients were isolated and identified by the bioassay-guided isolation.Further,the inhibition rates of root length and fresh weight of the two active ingredients were investigated on 19 weed species.Results showed that taiwanin E showed significant activities on the root length of Solanum nigrum L.and A. conyzoides and the IC₅₀ values of 7 d after the treatment were 6.84 and 8.76 μg/mL,respectively.The IC₅₀ values of 7 d after the treatment of taiwanin H on S. nigrum,Portulaca oleracea L.,Mikania micrantha Kunth and Drymaria diandra Blume were less than 10 μg/mL.As far as the fresh weight was concerned,the two active ingredients showed the best activity on A. conyzoides and the IC₅₀ values of 7 d after the treatment were 24.12 and 21.31 μg/mL,respectively.Generally,the two active ingredients were more active on the broadleaves than the grass weeds and taiwanin H showed a better activity than taiwanin E. Cytotoxicity studies revealed that taiwanin H could destroy the nucleus of the tobacco BY-2 cells and showed cytotoxicity.The herbicidal activities of taiwanins H and E,belonging to lignans,were firstly reported.