Abstract: Objective:To evaluate the role of miR-124 in breast cancer and its underlying mechanism. Methods:Quantitative re-verse transcription-polymerase chain reaction (qRT-PCR) was employed to quantify the expression level of miR- 124 in the breast can -cer cell lines and matched tissues of 52patients. Cell proliferation, invasion, and migration of MDA-MB-231 and T- 47D were deter-mined by miR-124 overexpressionin vitro . Luciferase vectors (pMIR-SP 1 3'UTR) were also constructed. The predicted target gene of miR-124 was identified via luciferase activation assay. The mRNA and protein expression of SP1 was detected via qRT-PCR and West -ern blot, respectively. Results: MiR-124 was decreased in breast cancer tissues and cell lines. This result is correlated with metastatic capacity, TMN stages, and prognosis in breast cancer tissues. In breast cancer cell lines, ectopic overexpression of miR-124 inhibited cell proliferation, invasion, and migrationin vitro . MiR- 124 mimics significantly inhibited luciferase activation (P<0.05) in HEK 293 cells and could significantly decrease the mRNA (P<0.05) and protein expression levels of SP 1 in MDA-MB- 231 and T- 47D cells.Con -clusion: MiR-124 could be inhibited in breast cancer. The low miR- 124 expression is associated with poor prognosis. In addition, miR-124 could inhibit cell proliferation, invasion, and migration by targeting SP 1. These findings confirm that miR- 124 downregulation may be a key mechanism for breast cancer carcinogenesis.