Background and Objective: Protein fibrils are deposited in different tissues as amyloid plaques, and it is strongly believed that they contribute to the pathogenesis of disorders, called amyloidosis. Cuminum cyminum has been used traditionally for dementia, which is a related disease to protein fibrillation. The purpose of this study was to investigate the inhibitory effects of Iranian herbal medicinal essential oils, as sources of small molecules, on the protein fibrillation process and their cytotoxicity effects. Materials and Methods: Fibrillation was characterized using circular dichroism spectroscopy, fluorescence assay with thioflavin T, and atomic force microscopy. Cytotoxicity studies were examined on SK-N-MC cell lines by MTT, lactate dehydrogenase and glucose consumption assays. Results: The study showed that the C. cyminum essential oil inhibited fibrillation of the hen egg white lysozyme (HEWL), representing the protein model for fibrillation. This effect was specific, although, other herbal medicinal oils did not demonstrate similar inhibitory effects on protein fibrillation, in fact, sometimes they induced fibrillation. Cytotoxicity assays indicated that the oil alone destroyed the cultivated cells. Conclusion: It is concluded that although C. cyminum have a significant effect on the inhibition of protein fibrillation but causes cell death.