Signal transducer and activator of transcription 3 (STAT3) protein frequently overexpressed in many malignancies and plays an essential role in regulating proliferation, apoptosis, migration and invasion in cancer cells. However, No STAT3 inhibitor was used clinically. In this study, we evaluated the toxic potential of a novel STAT3 inhibitor, 6Br-6a in breast cancer cell lines. The anti-cancer effect and underlying mechanism of 6Br-6a on MDA-MB-231 and MCF-7 cells were determined in vitro and in xenograft mouse model. Our data demonstrated that 6Br-6a significantly induced cell cycle arrest and cell apoptosis in breast cancer via blocking the activation of STAT3. Finally, we verified these inhibitory effects of 6Br-6a in the MDA-MB-231 xenograft mouse model. In conclusion, 6Br-6a effectively inhibited activation of STAT3 and induced cell cycle arrest and apoptosis via regulating cyclin D1 and Bcl-2 expression. All of these data indicate that 6Br-6a could be a potential candidate for the treatment of breast cancer.