Abstract: The volatile components of Conyza canadensis essential oil from steam distillation were analyzed by gas chromatography-mass spectrometry (GC-MS). There are five kinds of active components with match of more than 83%, of which four components are terpenoids (limonene, α-bergamotene, (E)-β-farnesene, (Z)-β-farnesene). The results of standard seed ger mination assay on paper showed that the essential oil of C.canadensis had inhibition effects on seed ger mination of the receptor plants (Brassica chinensis Linn., B.pekinensis Rupr., Triticum sestivum Linn., Sorghum bicolor(Linn.)Moench). The essential oil of C.canadensis on the receptor plants seed ger mination to 50% inhibition of the appropriate concentration of acetone IC50 were 0.4, 0.6, 0.4, 0.8 g/L, respectively. The active components of essential oil from C.canadensis are mainly terpenoids (limonene, etc.) and ketones (2,3-dimethyl-4(3H)-quinazolinone,etc.).