In this work, the effects of potassium nitrate (KNO3) on the seed germination, seedling growth (radicle length, radicle number and fresh weight), mitotic activity and chromosomal aberrations of Allium cepa L. germinated under both normal conditions and salt stress were studied. The final germination percentages, radicle lengths, radical numbers and fresh weights of onion seeds germinated in the medium with KNO3 alone were statistically the same as ones of the control seeds germinated in distilled water medium. In addition, the mitotic index in root tip meristems of A. cepa seeds germinated in the medium with KNO3 alone demonstrated a decrease according to ones of the control seeds germinated in distilled water medium, while their frequency of chromosomal aberrations showed an increase according to the control. On the other hand, salt stress considerably inhibited the seed germination and seedling growth of A. cepa. Furthermore, it markedly decreased the mitotic index in root tip meristems of the seeds and increased the number of chromosomal aberrations. The detrimental effects of salt on the seed germination, seedling growth, mitotic activity and chromosomal aberrations was dramatically alleviated in varying degrees by KNO3 application.