MicroRNAs (miRNAs) are small non-coding RNAs that, by targeting certain messenger RNAs (mRNAs) for translational repression or cleavage, can regulate the expression of these genes. In addition, miRNAs may also function as oncogenes and tumor-suppressor genes, as the abnormal expression of miRNAs is associated with various human tumors. However, the effects of the expression of miR-124 in breast cancer remain unclear. The present study was conducted to study the expression of miR-124 in breast cancer, paying particular attention to miR-124's relation to the proliferation, invasion, and apoptosis in breast cancer cell MCF-7 and MDA-MB-231. Real-time quantitative RT-PCR (qRT-PCR) was performed to identify miR-124 that was down-regulated in breast cancer tissues. We also showed E26 transformation specific-1 (Ets-1) and miR-124 expression levels in breast cancer tissues that were associated with lymph node metastases. With transfected synthetic miR-124 agomir into MCF-7 and MDA-MB-231, a significant reduction (P < 0.05) in MCF-7 and MDA-MB-231 cell proliferation and colony forming potential was observed after treatment with miR-124. Apoptosis and migration rates were found to be significantly higher in two breast-derived cell lines transfected with a miR-124 agomir (P < 0.05). Luciferase reporter assay and Western blot were used to verify Ets-1 as a potential major target gene of miR-124, and the result showed that miR-124 can bind to putative binding sites within the Ets-1 mRNA 3' untranslated region (UTR) to reduce its expression. Based on these findings, we propose that miR-124 and Ets-1 may serve as a therapeutic agent in breast cancer.