Nerium oleander known as oleander has belonged to the poisonous plants its habitat in a tropical andsubtropical region. The chemical analysis with GC-Mass of the alcoholic extract of oleander leaves revealedthat this plant has many chemical compounds more than 80 compounds and high-peaks about 29 compoundswhich are represented by alkaloids, phenol, terpenes, and fatty acid. HPLC analysis showed many essentialoils that have many biological effects.To evaluate the antibacterial activity of the alcoholic extract of N. oleander against locally isolatedPseudomonas aeroginosa the broth micro-dilution method was adapted to different concentrations werestarted from 3.9 to1000 mg/ml. The results revealed that the alcoholic extract has antibacterial activitydepending on the concentration and bacterial strain.When treating the onion root cells as a model of the study to find the cellular toxic effect the results showedthat treatment with an alcoholic extract of oleander leaves with concentrations (control, 150, 300, 600,and 1200 mg /ml) for four hrs., the mitotic index (MI) was decreased to less than 50%, when treated withhigh concentrations 600,1200 mg /ml and many cases of chromosomal aberration were shown, such as theabnormal shape of the cells with a percentage about 1 and 2.8 % at the concentrations 600 and 1200 mg /ml respectively, as well as the multinucleated cells recorded the highest percentage at the concentrations1200 mg /ml with 14.4%, and nuclear lesion (chromatin degradation) were seen in a large percentage atthe interphase were recorded 13.66, 25.42, and 32.3% at the concentrations 300,600, and1200 mg /mlrespectively.