The study was aimed to explore the phytotoxic potential of Sonchus oleraceus L. on the emergence and seedling growth of Echinocloa cruss-galli L. and quantification of allelochemicals present in leaves, stem and fruit of S. oleraceus weed. The experiment comprised of aqueous extracts of leaf, roots and fruits applied at 0.25, 0.5, 1, 2, 4 and 8% (w/v) along with a control (0%). Data regarding seed emergence, emergence index, emergence percentage, root length, shoot length and seedling dry weight were recorded. All the tested concentrations of all the parts of S. oleraceus significantly inhibited the seed emergence, emergence index, emergence percentage (%), growth and delayed mean emergence time and time taken to 50% emergence of E. cruss-galli. However, maximum mean emergence time (5.20 days) and time taken to 50% emergence (3.49 days), were noted at 8% concentration of fruit and leaf aqueous extracts, respectively. Fruit aqueous extract at 8% concentration caused the lowest germination index (1.70), germination percentage (40%), root length (0.09 cm), shoot length (0.45 cm) and dry weight (0.16 g) of E. cruss-galli. Results suggested that the extract of S. oleraceus weed at concentration (8%) can be used as potential bio-herbicide for the control of E. cruss-galli weed.