Abstract: Allelopathic effects of water extract from Xanthium sibiricum were tested with a petri dish cultivation method. The extract strongly inhibited germination, and seedling growth of sorghum(Sorghum vulgare), cucumber (Cucumis sativus), wheat(Triticum aestivum), rape(Brassica campestris), and radish (Raphanus sativus). The inhibition of rape was the strongest followed by radish. Roots were inhibited more strongly than shoots at the same concentrations of the extract. Allelopathic effects of extract on photosynthesis activity and malondialdehyde (MDA) content were also tested. The inhibitions of X. sibiricum extract to embryo and root of endosperm-removed wheat were 58.33%, 67.31% at 0.025 0 g/mL respectively, while they were 5.58%, 32.34% to sorghum cultivated in the dark. The MDA contents of cucumber and radish increased after treatment with X. sibiricum extract. These Results indicate that the allelopathic effects of X. sibiricum could inhibit photosynthesis activity and MDA, and could also inhibit non-photosynthesis activity to some extent.