To study chemical constituents of the flowers of Eupatorium japonicum.Nineteen known compounds were isolated from the 70% ethanol extraction of the flowers of E.japonicum by various chromatographic methods.On the basis of NMR and MS spectroscopic analysis,their structures were identified as coumarin (1),2-hydroxy-2,6-dimethylbenzofuran-3(2H)-one (2),1-(2-hydroxy-4-methylphenyl)propan-1,2-dione (3),subamone (4),(7R*)-opposit-4(15)-ene-1β,7-diol (5),gentisic acid (6),caffeic acid (7),9-hydroxythymol (8),protocatechuic acid (9),trans-o-hydroxycinnamic acid (10),umbelliferone (11),quercetin (12),traxasterol (13),9-acetoxythymol 3-O-tiglate (14),taraxasterol acetate(15),linoleic acid(16),9-angeloyloxythymol(17),stigmasterol(18),palmitic acid(19).Compounds 2-9,11-13,16,19 were isolated from E. japonicum for the first time,and compounds 4-6,16 were isolated from the genus of Eupatorium for the first time.