Background: Breast cancer is considered as one of the most influential diseases around the world. Radiation is one of many ways that is being use to against cancer. However, patients treated with radiation may face cancer recurrence. Some plant compounds exhibit antioxidant effects. Sinensetin is a methylated flavone present in citrus and Orthosiphon stamineus. In this study, we examined the role of sinensetin in increasing the radiation sensitivity. Method: The cytotoxic effect of sinensetin was evaluated in MDA-MB-231 by MTT assay. Additionally, the clonogenic ability of cells was evaluated in the presence of sinensetin. Real-Time PCR was performed to detect and quantify expression profiles of apoptosis related genes. Result: Sinensetin decreased the viability of MDA-MB-231 in a concentration and time dependent manner. The survival fraction was decreased in cells treated with sinensetin prior to X-irradiation compared to cells treated with X-ray only. Furthermore, treatment of cells with sinensetin and X-ray could increase expression level of p53, Bcl-2 and STAT3. Conclusion: According to the results, sinensetin combined with X-ray can induce apoptosis in the treated cells.