Objective: To investigate miRNA-639 (miR-639) expression in breast cancer and its relations with growth, invasion and metastasis of the breast cancer. Methods: The miR-639 expression in 60 specimens of breast cancer tissue (30 metastatic cases and 30 non-metastatic cases) and 30 specimens of tumor adjacent tissue, along with human breast cancer MD231 and MCF-7 cells were detected by using fluorescent quantitative PCR. The relationship between miR-639 expression level and clinicopathologic features of breast cancer patients was analyzed. The effects of miR-639 on the proliferation, migration and invasion of breast cancer were evaluated by CCK-8 assay, wound scratch assay and Boyden chamber assays, respectively. Results: MiR-639 expression level in breast cancer tissues was significantly higher than that in adjacent tissue, in metastatic breast cancer tissue was significantly higher than that in non-metastatic tissue, and in highly invasive MD231 cells was significantly higher than that in low invasive MCF-7 cells, with all differences reaching statistical significance (all P<0.05). Among the studied clinicopathologic factors, the miR-639 expression was only associated with the M stage of the breast cancer patients (P<0.01). The proliferative, migratory and invasive abilities of MD231 cells were significantly decreased after miR-639 inhibitor transfection, whereas in MCF-7 cells they were significantly increased after miR-639 transfection (all P<0.05). Conclusion: MiR-639 expression is elevated in breast cancer, and the abilities of proliferation, migration and invasion of breast cancer are increased proportionately with the level of miR-639 expression.