Objective：To investigate the effects of STA-21，a specific inhibitor of signal transducers and activators of transcription 3（STAT3） on the proliferation and apoptosis of breast cancer MCF-7B cells. Methods：MTT assay was applied to observe the inhibi－tion of STA-21 on growth of MCF-7B cells after MCF-7B cells being treated with different concentrations of STA-21. Apoptosis and cell cycle of MCF-7B cells were examined by flow cytometry（FCM）. Expressions of P-STAT3，STAT3，CyclinD1 and Bcl-xl protein in MCF-7B cells were detected by Western blot. Results：After cells being exposed to different concentrations（10，30，50 μmol/L）of STA-21，inhibitory rate and apoptotic rate were increased markedly（P=0.000）. It was showed by FCM assay that STA-21 could inhibit the MCF-7B cells at G1 phase. Meanwhile，STA-21 could reduce the expressions of P-STAT3，CyclinD1 and Bcl-xl protein（P=0.000），but STA-21 exerted no effect on STAT3 protein（P=0.367）. Conclusions：Our study demonstrates that STA-21 can inhibit prolifer－ation and promote apoptosis of MCF-7B，mechanism of which may be related with the inhibition of STAT3 activity.