Interference of Sonchus oleraceus was studied by evaluating its competitive and allelopathic potential on three weed species: Brassica nigra, Chenopodium murale and Melilotus indicus. The allelopathic potential of S. oleraceus was assessed using the S. oleraceus-infested soil collected from its dominating communities and its root exudates trapped using living S. oleraceus, while its competitive ability was assessed in greenhouse in an additive design with ratio 1:1 at different densities. The rhizosphere soil of S. oleraceus inhibited germination and growth of B. nigra and M. indicus. Chlorophyll content of the target weeds was not affected in this test. Soils containing living S. oleraceus inhibited germination and growth of all weeds except germination of C. murale. Interspecific competition reduced germination of C. murale and M. indicus, and increased mortality of the former at all densities. HPLC analyses for soil samples indicated the presence of several phenolic compounds among which caffeic acid and catechol were abundant in the S. oleraceus-infested soil while caffeic and ferulic acids were abundant in its root exudates. The data suggest that both allelopathic and competitive potential of S. oleraceus function, and its interference plays at least a partial role in the community organization of weeds in agroecosystem.