The total amount of monoterpenoids in A. artemisiifolia var. elatior leaves was increased continuously after Apr. and the highest amount was in Jul. The major monoterpenoids in A. artemisiifolia var. elatior leaves were consisted of 29 compounds and were dominated by No. 2, octene, α -pinene, β -myrcene ,1,3,6-octatrine, endo-borneol and (-)-bornyl acetate. The allelopathic effects of A. artemisiifolia var elatior leaves on the roots and shoots elongation of the selected plants were studied and there were significant allelopathic effects. The effects were much different between the selected plants and the effects signifcantly became higher with the quantity of leaves used in the experiment. The Lactuca sativa, Achyranthes japonica, and Oenothera odorta were suffered much higher effects than the other species. The allelopathic effects were also different with the leaves sampled in different seasons and the effects in july were found a little higher than in June and May.