Abstract:In order to ascertain the allelopathy of Parthenium hysterophorus L. flower on Echinochloa crusgalli (L.) Beauv. and allelochemicals, the allelopathic effects of ethyl acetate fraction, n-butanol fraction and residuary water fraction of P.hysterophorus flower aqueous extract on the growth of E.crusgalli were studied by the way of indoor Petri dish bioassay and indoor pot. The results in Petri dish and greenhouse showed ethyl acetate fraction had the highest bioactivity, and the inhibition of germination, plant height and fresh weight in greenhouse were 11.1%, 10.2% and 14.6% for E.crusgalli respectively at 50g/L of aqueous extract. Fifteen compounds were identified in ethyl acetate fraction by the method of GC-MS, and the constituents are mainly hydrocarbon and its derivatives, naphthalenes, alcohols, benzenes, acids, ketones and esters.