Aerial parts of Xanthium italicum in an air tight container greatly inhibited root elongation of radish,implying that this invasive plant could release biologically active volatile organic compounds(VOCs)into the environment to affect other plants’ growth.This phenomenon was further studied by evaluating the phytotoxic effects of X.italicum essential oil against two dicot plants,amaranth(Amaranthus mangostanus L.)and lettuce(Lectuca sativa L.),and two monocot plants,wheat(Triticum aestivum Linn)and ryegrass(Lolium multiforum),and analyzing the chemical composition of the oil.Among the 4 test species,amaranth was the most sensitive plant,0.5μl/mL essential oil application resulted in a 50% reduction on root elongation,and 2.5 μl/mL essential oil almost completely inhibited its seedling growth.Wheat was the least sensitive species,whose root growth was reduced to 36% of control by 5 μl/mL essential oil.The essential oil exerted moderate inhibitory effect on both lettuce and ryegrass.Compared to a commercial herbicide-Harness,X.italicum oil exhibited stronger phytotoxicity on amaranth,lettuce and wheat,but weaker activity on ryegrass.The chemical composition of the essential oil isolated by hydrodistillation from the aerial parts of X.italicum Moretti was analyzed by GC/MS.Thirty two compounds were identified,representing 94.89% of total oil,which was found to be rich in monoterpene hydrocarbons(60.71%).The main constituents of the oil were limonene(51.61%),germacrene B(6.98%),δ-cadinol(5.94%),β-pinene(5.23%),α-caryophyllene(5.1%)and bornyl acetate(3.15%).Bioassay revealed the dominant constituent-limonene,was unlikely the responsible phytotoxic compound due to its low biological activity;rather,there might be other oil constituent(s)that either act alone,or work together,and possibly assisted by synergistic effect,to display the phytotoxic activity.Our results suggested that X.italicum might produce allelopathic VOCs to facilitate its invasion success.This is the first report on the phytotoxic activity and the chemical composition of the essential oil of X.italicum Moretti from China. Aerial parts of Xanthium italicum in an air tight container greatly inhibited root elongation of radish, implying that this invasive plant could release biologically active volatile organic compounds (VOCs) into the environment to affect other plants’ growth. This phenomenon was further studied by evaluating the phytotoxic effects of X.italicum essential oil against two dicot plants, amaranth (Amaranthus mangostanus L.) and lettuce (Lectuca sativa L.), and two monocot plants, wheat (Triticum aestivum Linn) and ryegrass (Lolium multiforum), and analyzing the chemical composition of the oil. Among the 4 test species, amaranth was the most sensitive plant, 0.5 μl / mL essential oil application resulted in a 50% reduction on root elongation, and 2.5 μl / mL essential oil almost completely inhibited its seedling growth. Wheat was the least sensitive species, whose root growth was reduced to 36% of control by 5 μl / mL essential oil. Essential oil exerted moderate inhibitory effect on both lettuce and ryegrass. C ompared to a commercial herbicide-Harness, X.italicum oil stronger in phytotoxicity on amaranth, lettuce and wheat, but weaker activity on ryegrass. The chemical composition of the essential oil isolated by hydrodistillation from the aerial parts of X.italicum Moretti was analyzed by The main constituents of the oil were limonene (51.61%), germacrene B (6.98%). The main constituents of the oil were limonene , δ-cadinol (5.94%), β-pinene (5.23%), α-caryophyllene (5.1%) and bornyl acetate (3.15%). Bioassay revealed the dominant constituent-limonene, was unlikely the responsible phytotoxic compound due to its low biological activity; rather, there might be other oil constituent (s) that either act alone or or work together, and possibly assisted by synergistic effect, to display the phytotoxic activity. Our results suggested that X.italicum might produce allelopathic VOCs to develop its invasion success.This is th e first report on the phytotoxic activity and the chemical composition of the essential oil of X.italicum Moretti from China.