Brassica campestris is a typical daily crop grown all over the world as a source of fodder, vegetables, and oil. The allelopathic effects of Populus nigra L. and Melia azedarach L. extracted leaves on seed germination and growth performance of Brassica campestris L. were studied in the field and in the laboratory. Seeds of Brassica campestris were seeded in pots in the field, whereas seeds sown in tap water were considered as a control. Different leave weight, of both P. nigra L. and M. azedarach was used and mixed with soil separately, to test their allopathic effects. Growth parameters including stem height, stem diameter, leaf number, number of internodes, internodal length, fresh and dry weight of the stem, and 1000 seed-weight were investigated. Number of flowers per plant; and inflorescence size were also considered. The results revealed that all measured parameters of B. campestris were negatively influenced, with the exception of internodal length, which was found to be positively affected. Seeds treated with plant extracts showed an inhibitory effect on seed germination which was directly proportional with doses of leaves weight used. In laboratory experiments, the allelopathic effects of P. nigra aqueous extract on seed germination (%) as well as growth performance of germinated seed, expressed as plumule length and radicle length, of B. campestris were reported. Parallel to the field experiment, a high dose of leaf weight extracted from dried leaves greatly decreased seed germination (%) and growth measured parameters, which are proportional to extracted leaf weight.