This study aimed to investigate the cytogenetic effects of two plant plant growth regulator as, naphthalene acetic acid(NAA)and benzylaminopurine(BAP)in root meristematic cells of onion.The categories of mitotic index rate , as well as, chromosomal aberrations and types of metaphase disturbance were comparatively analyzed after germinated the onion in different concentrations of each hormone.Both hormones stimulated cell division and have aneugenic potential by acting as spindle posion via disturbed the correct separation of chromosomes to the opposite cell poles.The highest percentage of irregular cell shape was induced by five ppm of NAA.The treatments of BAP appeared a dose – response for increasing the percentage of extended cells.All treatments increased the percentage of chromosomal abnormalities above the control group.The most dominant phenotypes of abnormalities were binucleate cells, ghost cells, vacuolated nuclei, necrosis and extended cells, which reflected the toxicity of phytohormones leading to cell death.The most dominant cytological irregularities were c-mitosis, diagonal cells, bridges at ana-telophase, chromosomal breaks and stickness which indicates clastogenic potential of these hormones.Abnormalities induced reflected the action of stimulators on the spindle apparatus such as c-metaphase, laggards chromosomes and multipolar of ana-telophases.All treatments changed the percentage of mitotic index such decreased by NAA or increased by BAP in relation to control.The decline in mitotic index reflected the inhibition of DNA synthesis. Prophase cells declined, meanwhile metaphase and anaphase cells were increased in relation to control.It can be concluded that phytohormones caused toxic effects on the eukaryotic cells via induced different types of irregular mitosis as shown in this study.