The aim of this study is to investigate the protective effect of lichen Cladonia foliacae (Huds.) (CF) on hydrogen peroxide (H2O2)-induced toxicity through cell death, chromosome aberrations, mitotic index, oxidative stress parameters, and DNA damage in a Allium cepa root meristematic cells. Any chemical was not given for control group. Two doses of H2O2 (3 and 7%) were given to the roots for 1h and the root tips were treated with CF water extract (50 and 100L) with increasing times for treatment groups. The roots were taken from control and treatment groups, and mitotic index, cell death, and chromosome aberrations were performed by light microscope. Changing antioxidant capacity of roots was revealed by FRAP and TEAC assay. Also, DNA damage was measured by comet assay and RAPD-PCR technique. Chromosome aberration values were obtained with increasing concentrations with longer treatment times, such as chromosome bridge, vagrant, and polyploidy in both groups. Increasing exposure doses of H2O2 caused decreasing mitotic index values at 72h. TEAC and FRAP assay demonstrated that roots' capacity of antioxidant was altered by increasing concentrations of H2O2. The tail DNA% and tail length significantly increased in all exposure times when compared to control group. Three and seven percent of H2O2 caused the genotoxic effect on genetic material at 72h according to RAPD-PCR technique. Increasing the doses of H2O2 resulted in increased toxicity to all studied parameters of A. cepa, but CF extract altered all changing parameters of A. cepa root cell. The H2O2 tested in this study have cytotoxic and mutagenic potential, but extract of CF was protective against H2O2 caused toxicological changes. But, it did not protect completely in the A. cepa root meristematic cells.