Abstract： Objective To explore the effect of hyperbaric oxygen (HBO) on metastasis of breast cancer cells through down-regulation of the expression of hypoxia-induced factor-1α (HIF-1 α),so as to provide new treatment strategy for breast cancer.Methods The breast cancer cells MCF-7 were divided into the blank control group,the cobalt chloride (CoCl2) treatment group,the CoCl2 + HBO group,and the CoCl2 treatment + HBO + HIF-1 α over-expression group.CoCl2 was used to simulate the hypoxic environment.Western blotting was performed to detect the expressions of HIF-1α,Bcl-2 and Bax.CCK-8,Flow cytometry,wound healing and Transwell chamber experiment were also performed to evaluate the growth,apoptosis,migration and invasion of MCF-7 cells in different groups.Results Following CoCl2 treatment,the expressions levels of HIF-1α and Bcl-2 all elevated significantly in MCF-7 cells,while the expression level of Bax decreased (P ＜ 0.05).Cell growth,metastasis and invasion were markedly enhanced,while cell apoptosis was significantly inhibited(P ＜0.05).As compared with that of the CoCl2 treatment group,HIF-1α expression level of the CoCl2 + HBO group decreased,and cell growth,metastasis and invasion were markedly suppressed,while cell apoptosis was significantly enhanced.Following over-expression of HIF-1α,the effect of HBO on cell growth,metastasis and invasion was inhibited considerably (P ＜ 0.05).Conclusion Hyperbaric oxygen inhibited the growth,metastasis and invasion of breast cancer cells MCF-7,and at the same time promoted cell apoptosis through down-regulation of the expression of HIF-1α.