The present study was carried out to evaluate the allelopathic potential of ten common weeds against three crop plants. All weeds extracts, even those more diluted, completely prevented seeds of Eruca sativa from germination. The high extract strength of Ammi majus and Desmostachya bipinnata prevented seeds of Triticum aestivum and Vicia faba from germination. The germination percentage, seed vigor index, coefficient of velocity and seedling length of T. aestivum and V. fabadifferentially inhibited by the extracts of weeds. The rate of elongation of hypocotyl and epicotyl of T. aestivum inhibited by all weeds, while the low extract strength of six weeds stimulated the rate of elongation in V. faba sprouts. All estimated germination and elongation parameters of receiving plants negatively correlated by total phenolics, flavonoids and alkaloids in donor weeds. Terpenoids were less influence and weakly correlated with germination parameters, so it suggested to be stimulatory. The magnitude of allelopathic effect, inhibitor or stimulator, was primarily depends on the donor plant and its content of secondary metabolites and secondarily on the target species as indicated by 2. The weeds exerting negative allelopathy can be categorized into competitive weeds which inhibit cell division and elongation or phytotoxic weeds that germination-preventing.