Breast cancer is a malignant tumor from normal breast epithelial. In recent years, many literature reports sought to determine the expression of predicted target genes of microRNA and their potential function, pathways and networks, which are involved in the tumorigenesis, metastasis and prognosis of breast cancer. The miR-21 has recently been found to be highly expressed in solid tumors than normal tissue, and it has exposed some layers of gene expression regulation that becomes a hot topic of breast cancer. This paper briefly reviews advances in research on miR-21 in breast cancer.