Johnsongrass (Sorghum halepense (L.) Pers.) is a troublesome weed species of many crops in Turkey as well as worldwide. Allelopathic potential of residues of some brassica species, which are round white radish (Raphanus sativus L.), garden radish ( R. sativus L.), black radish ( R. sativus L. var. niger), little radish ( R. sativus L. var. radicula, turnip ( Brassica campestris L. subsp. rapa) and rapeseed ( Brassica napus L. oleifera DC.) on johnsongrass were investigated under both laboratory and field conditions. All species suppressed johnsongrass in field and laboratory conditions. The lowest suppression was from garden radish, which has already been used to control johnsongrass by few farmers in Turkey. It is concluded that the plants studied can be used to control johnsongrass. Higher amount of isothiocyanates ( isothiocyanate benzyl, Isothiocyanate allyl) in black radish extract and lower amount of isothiocyanates at garden radish extract were determined. Parallel results for johnsongrass suppression and amount of isothiocyanates show that allelopathy play roles in johnsongrass suppression by brassica species.