Abstract: In the screening of resources with herbicidal activity, a fungus Xylaria sp.NH1801 with significant herbicidal activity was obtained. The active compound of Xylaria sp. NH 1801 were traced and isolated and two compounds were identified as 19,20-epoxy-cytochalasin Q（1） and 21-deacetylated-19,20-epoxy-cytochalasin Q（2）. At 50 μg/mL, compounds 1 and 2 significantly inhibition toward the root elongation of Echinochloa crusgalli（inhibition rates were 65.7% and 60.7%, respectively） and Amaranthus retroflexus seeds （inhibition rates were 77.2% and 75.4%, respectively）. This is the first report on the inhibitory effect of compounds 1 and 2 on Echinochloa crusgalli and Amaranthus retroflexus seeds.

Key words: Xylaria sp., herbicidal, cytochalasin, 19, 20-epoxy-cytochalasin