The allelopathic potentials of volatiles and liquid residue of Ageratum conyzoides L. on Echinochloa crusgalli L., Lolium perenne L. and Bidens pilosa L. were investigated. The results showed that the volatiles released from leaves and stems of A. conyzoides in an airproof system significantly inhibited the seedling growth of E. crusgalli, L. perenne and B. pilosa. Compared to the control, root length, shoot length and fresh weight of E. crusgalli were decreased by 7.2%, 8.3% and 7.7%, respectively, of L. perenne by 13.4%, 22.7% and 7.7%, respectively, and of B. pilosa by 20.0%, 14.7% and 33.3%, respectively. Liquid residue of A. conyzoides also significantly inhibited the seedling growth of the three tested weed species and the inhibition was enhanced with increasing concentration. When liquid residue concentrations were 0.01 g/mL, 0.10 g/mL and 1.00 g/mL, root length of E. crusgalli was decreased by 44.6%, 91.6% and 100.0%, respectively, of L. perenne by 26.8%, 65.8% and 100.0%, respectively, and of B. pilosa by 26.0%, 68.5% and 100%, respectively; shoot length of E. crusgalli was decreased by 48.5%, 78.1% and 98.7%, respectively, of L. perenne by 25.1%, 69.1% and 100.0%, respectively, and of B. pilosa by 19.8%, 46.8% and 100.0%, respectively; fresh weight of E. crusgalli was decreased by 41.7%, 58.3% and 83.3%, respectively, of L. perenne by 12.9%, 48.4% and 100.0%, respectively, and of B. pilosa by 28.6%, 57.1% and 100.0%, respectively, compared to the control.