Phenolic compounds, which are products of secondary metabolism, have been demonstrated to be widespread growth substances in plants. The objectives of this study were to identify the phenolic compounds in common ragweed (Ambrosia artemisiifolia var. elatior) by HPLC and to evaluate their effects on germination and seedling growth of three weed species. Under controlled conditions in Petri dishes at $25^{circ}C$, $10^{-3}$ and $10^{-4}$ M solutions of phenolic compounds were evaluated in seed germination tests. Four phenolic compounds (caffeic acid, O-coumaric acid, ${ ho}$-coumaric acid and ferulic acid) in common ragweed plant were identified and their concentration was increased from the stage before flowering through full flowering stage. Treatment of O- and ${ ho}$-coumaric acids delayed the seed germination of Digitalia ciliaris, while the treatment of caffeic acid delayed the seed germination of Echinochloa crus-galli. In time to 50% germination ($T_{50}$), phenolic compounds at $10^{-4}$ M promoted in Cyperus microiria and E. crus-galli but the level of $10^{-3}$ M delayed the $T_{50}$ of those weeds. The O-coumaric acid inhibited seed germination and seedling growth of the tested weeds and especially it perfectly inhibited the root growth of E. crus-galli.