Matrix metallopmteinase 9(MMP-9) is one kind of matrix metalloproteinases and plays a role in the process of breast cancer metastasis. It can not only degrade the basement membrane and the extracellular matrix around tumor tissue but also promote angiogenesis, even create conditions for the growth of tumors, local invasion and distant metastasis. Current study shows that the over-expression of MMP-9 can significantly speed up the breast lymph node metastasis and the relevant research will provide guidance for clinical treatment and prognosis of breast cancer.