Abstract: Abstract: Allelopathy plays an important role in plants invasion. In this study we determined the effects of aqueous extract of root, stem, leaf and withered leaf from Oenothera laciniata on seed germination and seedling growth in two herbaceous species （Triticum aestivum and Brassicav chinensis）competing directly with it during the early stage of its invasion, and two crops（Conyza canadensis and Veronica persica）. The intensity of allelopathy effect and the affecting to crops were discussed.The relationships between allelopathy and invasiveness were discussed too .The results suggested that Oenothera laciniata has allelochemicals, different concentrations in different plant parts, which comprise most in leaves and root, fewer in stem, and the withered leaves have the fewest. The aqueous extract of Oenothera laciniata could inhibit seed germination and seedling growth in all the four species, and its effect was concentration dependent, increasing with the increase of concentration。The high concentration extract could reduce seed germination rate, germnation speed and radicle growth. Seed germination speed and radicle growth were more sensitive to the extract than others, which might be the most sensitive index to judge allelopathy, among parameters mentioned above. Sensitivity to the aqueous extract was different among the species tested in this study, germination of Veronica persica and seedling growth of Brassica chinensis were the most sensitive，and Triticum aestivum was unsusceptible to extract of Oenothera laciniata.