Background and purpose: Recently studies have shown that main compound (Cyclotides) of Viola odorata has an inhibitory effect against proliferation of cancer cells. Also, melatonin reduces size and growth of tumor cells. The main purpose of this study was to compare the inhibitory effect of Viola odorata and melatonin on MDA-MB-468 cells proliferation. Materials and methods: In this exprimental study, triple negative human breast cancer cells (MDA-MB-468) were cultured in 96-well plates and incubated with different concentrations of Viola odorata hydroalcoholic extract and melatonin for 24h. Cell viability was measured using MTT assay. In the in vivo study Balb/c mice (6 group, n=6) received subcutaneous injection of 100 µl of cell suspension (4T1 cells) in the left hind flank. The animals were treated with different concentrations of Viola odorata extract (50, 150, 250, and mg/kg) and melatonin (40 mg/kg) for three weeks. Tumor volume changes were measured weekly and weight changes were measured at day 21. Results: The in vitro study showed that different concentrations (30, 40, 60, 80,100,150, and 200 µg/ml) of Viola odorata significantly decreased cell survival in cancer cells (P<0.05). Also, melatonin at concentration of 1mM significantly decreased cell viability (P<0.05). The in vivo study found that melatonin (40 mg/kg) and Viola odorata at 250 mg/ml concentration inhibited increasing of tumor volume after 21 days compared with control group. Conclusion: Treatment with melatonin and Viola odorata extract was found effective in reducing cell survival and tumor volume.