Abstract: The sensitizing effect of caffeine to carbon ion radiation was investigated and the change of BRCA1 expression was observed. The MCF-7 breast carcinoma cells were exposed to carbon ion beams with or without caffeine. The cell survival was automatically monitored by RT-CES system. Cell cycle distribution was assessed by flow cytometry. The levels of BRCA1 mRNA were analyzed by real-time RTPCR. The expression of BRCA1 protein and its phosphorylation were examined by Western blot. The results show that caffeine increases the sensitivity of MCF-7 cells to carbon ion radiation, and abrogates the radiation-induced G2 arrest. Caffeine inhibits radiation-induced BRCA1 expression both at mRNA and protein level. At the same time, caffeine specifically abolishes BRCA1 phosphorylation of Ser-1524. The data implicate that caffeine inhibits the expression of BRCA1 protein and its phosphorylation.