Objective To observe the effects of tetra-atsenic oxide (As4O6) on the proliferation, migration and invasion of human breast cancer MCF-7 cells; To explore its potential mechanism. Methods The human breast cancer MCF-7 cells were regarded as the research object and cultured in vitro, with different concentrations of As4O6 using to intervene in MCF-7 cells. The cell proliferation was detected by MTT assay; the flow cytometry and wound-healing assay were used to detect the cell apoptosis and migration, respectively. The expressions of Cyclin E1, Cyclin A2, Caspase 3, p21 and MMP-9 mRNA were accessed by semi quantitative RT-PCR. Results As4O6 had a significant inhibitory effect on the proliferation of MCF-7 cells in a dose dependent manner. Compared with the control group (0 μmol/L), the apoptosis rate increased significantly when the concentration of As4O6 was 9, 12, 15 μmol/L (P<0.01). Either As4O6 at high (3 μmol/L) or low (1 μmol/L) concentration could effectively inhibit the migration of MCF-7 cells (P<0.01). With the increasing concentration of As4O6, the expressions of Cyclin E1, Caspase 3, and p21 mRNA significantly increased, while the expressions of Cyclin A2 and MMP-9 mRNA significantly decreased (P<0.05, P<0.01). Conclusion As4O6 can significantly inhibit the proliferation, cycle, invasion and migration of breast cancer MCF-7 cells, and the mechanism may be related to the increase of expressions of cyclin E1, caspase 3, p21 and inhibition of expressions of cyclin A2 and MMP-9.