The present study carried out at Suleyman Demirel University, Faculty of Agriculture of the Department of Field Crops in 2010. The aim of the study was to examine allelopathic effects of oregano oil (Origanum onites L.) on germination and seedling development of amaranth (Amaranthus retroflexus L.), dock (Rumex crispus L.) and mustard (Sinapis arvensis L.) and crop plants [wheat (Triticum aestivum cv Gun 91), sunflower (Helianthus annuus cv. Sirena) and chickpea (Cicer arietinum)]. Seeds of these plants were germinated in petri and pot. Oregano oil doses for petri study was 0 (control), 3, 6, 10 and 20 mu l, for pot study was 0 (control), 0.5, 1.0, 2.0 and 4.0 mg kg(-1). The germination rate, root length, seedling length and dry matter in each germination condition were investigated. All parameters were significant (p<0.01). The germination rate of seeds in petri and pot at control application was determined as 16.0 and 46.7% for amaranth, 53.3 and 74.0% for dock, 56.0 and 72.0% for mustard, 89.3 and 92.0% for sunflower, 100.0 and 100.0% chickpea, 100.0 and 100.0% for wheat, respectively. The lowest oregano oil doses (3 mu l and 0.5 mg kg(-1)) in petri and pot study reduced, germination rate of dock, amaranth and mustard dramatically, germination rate of other plants did not change. The increased doses of oregano oil in petri and pot study negatively affected the root length of dock, amaranth and mustard. The seedling length of amaranth, mustard and dock at 0.5 mg kg(-1) dose was decreased compared to control. As a result, it was determined that oregano oil (3 mu l and 0.5 mg kg(-1)) at low doses significantly inhibited weed seed germination and was less damaging to crop plants seed germination.