To explore the effect of nanometer realgar affecting epithelial mesenchymal transition of the breast cancer MCF-7 cells and its possible mechanism. Methods:Breast cancer MCF-7 cells were cultured in vitroand and use different doses of nanometer realgar to intervene. A control group was made. To test the expression of E-cad and HIF-1α, RT-PCR to detected the expression of Snail mRNA. Results:Compared with the control group,the E-cad positive rates of the nanometer realgar group were higher than the control group(P<0.01); the HIF-1α positive rates of the nanometer realgar group were less than the control group(P<0.01);the control group, nanometer realgar low, medium and high dose groups, doxorubicin group of Snail mRNA relative expression levels were （52.16±3.87）, （30.10±2.48）, （47.08±3.45）, （41.42±4.35）, （36.07±2.45） respectively. Conclusion:Nanometer realgar can inhibit the migration and invasion of breast cancer MCF-7 cells, and it has anti-tumor metastasis. Nanometer realgar can resist tumor through reversing EMT by raising the level of E-cad and reducing the level of HIF-1α and Snail mRNA.