Statement of the Problem: Oral cancer is among the ten most common cancers worldwide. It affects the life quality of patients in many ways.Purpose: The aim of this study was to compare the effects of two different systemic doses of Viola Odorata syrup on the prevention of 4-Nitroquinoline-1-oxide (4-NQO) induced tongue dysplasia in rats.Materials and Method: Forty-eight male Wistar rats were divided into four groups of A, B, C and D. Group A served as the control group. The rats in groups B to D received 30 ppm of 4-NQO in drinking water for 12 weeks. Additionally, the rats in groups B and C received Viola Odorata syrup at doses of 15 and 5 ml/kg, respectively, 3 times a week. Body weights were measured three times a week. At the end, the rats were euthanized and the tongue was removed. Histological evaluations for carcinogenesis were carried out under a light microscope.Results: The mean body weight of the rats in groups B, C, and D were lower than that in group A (p< 0.01). After 12 weeks of treatment, microscopically no histological changes of the tongue base epithelia were observed in the control group. The rats in group B did not show severe dysplastic changes; only mild to moderate histological changes including hyperplasia and hyperkeratosis were evident. These incidences were significantly more apparent in groups C with moderate to severe changes (p< 0.05) and group D with severe dysplastic changes (p< 0.01). Almost all rats in group D had hyperplasia and manifested all of the stages of dysplasia.Conclusion: Viola Odorata extract has dose-dependent inhibitory effects on the development of tongue induced dysplasia.