In this study, the potential antimutagenic/cytoprotective effect of the oil extract (2.5 µg/ml) from Anethum graveolens L. (dill) on oxidative DNA damage was tested. The experiments were carried out to inhibit mutagenicity induced by potassium bromate (KBrO₃-9 g/l) in Allium sativum assay using mitotic activities and nuclear DNA contents. Results showed a clear reduction in root length in all KBrO₃-treated groups compared to control plants. Mitotic activity, measured by the mitotic index (MI), and mitotic stages were significantly decreased in meristematic cells after KBrO₃ application. The results showed a decrease in the mitotic index to about 0.8% at 24 hours compared to 23.9% in untreated plants. Analysis of the potential cytoprotective and genoprotective effects of dill extract on nuclear DNA contents showed a statistically significant increase in nuclear DNA content compared to KBrO₃-treated roots. Clearly, dill oil showed high performance in protecting mitotic cells against the mutagenic effects of KBrO₃ by increasing the MI and decreasing abnormal chromosomes.