In order to know the allelopathy effect of different organs of Alternanthera philoxeroides, seeds of Brassica chinensis were treated by water extracts of fresh root, stem, leave and flower of A. philoxeroides at differ-ent concentration in Petri dish. The results showed that the water extract of different parts of A. philoxeroides allpromoted the seed germination, root length, seedling height and fresh weight of B. chinensis at lower concentration,and inhibited them at higher concentrations. The seed germination, root length, seedling height and fresh weight of B. chinensis deceased with increasing of concentrations of the extracts from different parts of A. philoxeroides. Theroot length and seedling height of B. chinensis treated by water extracts from A. philoxeroides increased with timegoing, and the seed germination reached the maximum in the 2 or 3 day, and then decreased. The fresh weight de-creased gradually at higher concentrations, and rose gradually at lower concentrations. The root length of B. chinensis was influenced seriously by the water extract of A. philoxeroides, and the allelopathic effect index reached 0.73, but the index of the seedling height reached 0.29 cm only. The seed germination, root length, seedlingheight and fresh weight of B. chinensis were strongly inhibited by the water extract of A. philoxeroides root, andthat of A. philoxeroides flower was the worst.