Анотация: To understand the allelopathic effect of essential oil extracted from leaves of Eucalyptus globulus against two weeds, the experiments were laid out as a randomized complete design with factorial layout in two stages (laboratory and greenhouse) in khoy-Iran, during growing season of 2010. Studied factors were Eucalyptus essential oil concentrations (0.5, 2, 3.5 and 5 nl/ml-in laboratory and 0.25, 0.5, 0.75 and 1 v/v-in greenhouse) and weeds which included ` Amaranthus blitoides ` and ` Cynodon dactylon `. Results indicated that, with increasing of eucalypt oil concentration under laboratory and greenhouse conditions, Germination percentage, Germination rate, Radicle length, Plumule length, Seedling height, Primary root length and Primary pedicle length, significantly decreased. Among the weed species tested, the highest inhibitory effect of essential oil was observed in ` A. Blitoides `. Under laboratory condition, Eucalypt oil concentration decreased seedling height in ` A. Blitoides ` from 61.65 to 37.92 mm, whereas in pot study, eucalypt oil greatly suppressed the seedling height of ` A. Blitoides ` from 34.67 to 22.17 mm. Findings suggest that Eucalypt volatile essential oil possess strong inhibitory potential against weeds that could be exploited for weed management and can be recommended as a biological herbicide with the aim of weeds control