In this study, allelopathic effects of some essential oil plants (Carum carvi L., Coriandrum sativum L., Foeniculum vulgare Mill., Lavandula stoechas L., Mentha spicata L., Origanum onites L., Pimpinella anisum L., Rosmarinus officinalis L., Salvia officinalis L. and Thymbra spicata L.) were investigated against some common weed species (Alcea pallida Waldst. & Kit., Amaranthus retroflexus L., Centaurea salsotitialis L., Raphanus raphanistrum L., Rumex nepalensis Spreng., Sinapis arvensis L. and Sonchus oleraceus L.) that grow in field and horticultural crops. Different concentrations (3, 6, 10 and 20 mu l) of the essential oils were tested against weed seeds in vitro and their effect on germination was determined. Inhibitory effects of essential oils and concentrations were analysed with Freadman's test and they were found significant. Essential oils from Carum carvi, Mentha spicata, Origanum onites and Thymbra spicata showed high inhibitory effect against weed seeds at lower concentrations. The main components (carvacrol, thymol, carvone, limonene) of these four essential oils were tested for seed germination at four different concentrations (500, 250, 125 and 62.5 mu g/ ml) against the same weeds. Thymol, carvacrol and carvone showed high inhibition even at low concentrations against weed seeds. Only Alcea pallida seeds showed resistance against all essential oils and components.