Wastewater of the olive oil factory or olive olive mill wastewater occurs at the time of olive oil production and shows an important environmental contaminating feature due to the organic substances inside them and the toxicities of these substances to various biological systems. The toxicity of olive mill wastewater stems from the phenolic compounds which are included in olive mill wastewater. In our study, genotoxic effects of the olive mill wastewater on the Allium cepa root tip meristem cells have been examined. The onions were rooted at 0.1, 0.3 and 0.5% concentrations of the olive mill wastewater. It was observed that concentrations decreased the mitotic index compared to the control and only the decrease at 0.5% group was statistically significant. In the study, the abnormalities such as anaphase bridge, laggard chromosome, C-metaphase, stickness, irregular distribution and polar deviation, etc. were observed. In the study done with 1, 3 and 5% concentrations of the olive olive mill wastewater, was concluded that root extension was too little and even 5% concentration showed a toxic effect.