In laboratory bioassay, the traditionally prepared unrefined oil from mustard and water mixture (50% concentration) and different layers of this mixture were applied to yellow starthistle seeds (Centaurea solstitialis L.). The results indicated that, three layers (upper, middle and lower) of mustard oil + water mixture significantly reduced the germination of yellow starthistle. There was complete inhibition of germination in upper and middle layers of mustard oil + water mixture added to petri dishes. The mixtures (20% and 40% concentration) were applied as pre-emergence and post-emergence sprays on plants in greenhouse condiditons. There was no effect of pre-emergence application on the emergence of yellow starthistle seedlings. However, pre- emergence application, (1 or 2 sprays) of upper layer of 40 % mixture reduced the plant height, fresh and dry mass of yellow starthistle. This study showed the allelopathic effects of mustard oil+water mixture on this weed. It could be used in organic crop production, where yellow starthistle is problem or combined with other management methods.