Abstract:A field experiment was carried out to study the direct damage of Eupatorium adenophorum Spreng on eggplant yield and its economic infestation levels. Additive series experiment and curve fits were adopted to study the change of eggplant height and yield characteristics under different densities of E.adenophorum and the competitive mechanisms of light, nutrients and water between eggplant and the weed. The results showed that the eggplant height, fruit number and its yield were not affected when the densities of E.adenophorum ranged from 1 to 2 plants per m2, whereas, the eggplant height, fruit number and its yield decreased significantly when the density of the weed was more than 5 plants per m2. The main cause of the eggplant yield loss may be due to the plant height of E.adenophorum, the interference to the rate of light penetration and the accumulation of water and nutrition in E.adenophorum. The hyperbola model (1/y=-0.0108+0.2083/x) could well fit the relation between the eggplant yield loss (y) and the weed density (x). The economic infestation level for E.adenophorum in eggplant field was between the range of 0.192% and 1.282%. The economic threshold was 0.03-0.26 plants per m2.