Background: There is an increasing interest in identifying potent cancer preventive and therapeutic agents. Silymarin is a flavonoids complex extracted from the milk thistle (Silybum marianum L.) seeds. Silymarin was found clinically successful in the treatment of various liver diseases. Silymarin is in the focus of cancer researchers due to its high antioxidant properties. Objective: The present study was aimed to investigate the effect of silymarin on 4T1 cells and compared with taxol. Methods: 4T1 cell line (BALB/c mouse mammary tumors) was cultured in RPMI medium containing FBS 10%. Cells were incubated with 5% CO2 in presence of different concentration of silymarin (25-50-75-100-125 µg/ml), taxol (1.25-2.5-5-10-20 nM) and combination of silymarin and taxol separately for 24, 48 and 72 hours. Cell viability was assessed using trypan blue and MTT staining. The cells morphology was studied using fluorescent dye (Hochest, propidium iodide). Results: Silymarin and taxol showed significant cytotoxic effects on breast cancer cell in a dose and time dependent manner. Condensation and deformation of the nuclei were also observed similarly for both treatments. In combination treatment silymarin enhanced the sensitivity of 4T1 cells to taxol in all doses. Conclusion: The cytotoxic effect of silymarin on mouse mammary tumors was comparable to taxol cytotoxicity. Treatment cells with combination of silymarin and taxol improved the cytotoxic effect of taxol.