〔Abstract〕 Objective To investigate the inhibitory effect of microRNA(miR)-100 on breast cells and breast cancer cell proliferation. Methods Immunohistochemistry and q-PCR were used to detect expressional difference of miR-100 among clinic breast carcinoma specimen cells, normal breast epithelial MCF-10 A cells and MCF-7 cells in breast cancer. miR-100 knock-down MCF-10 A cell line and miR-100 over-expression MCF-7 cell line were established by stable transfection of lentiviral vector of miR-100 inhibitor or lentiviral vector of miR-100 mimics. The cell activity and colony formation ability of the influence of miR-100 were compared between normal breast epithelial cells and breast cancer cells. Results MiR-100 expression in normal mammary tissues and cell lines evidently surpassed those in breast cancer tissues and cells. MiR-100 stable knock-down MCF-10 A cell line and miR-100 stable over-expression MCF-7 cell line were established and tested. It showed that the proliferation of breast cells was up-regulated after inhibiting its expression. On the contrary,the up-regulation of its expression could inhibit the malignant behavior of breast cancer cells,which was manifested by the decrease of proliferation level. Conclusion The expression of miR-100 is negatively correlated with the malignant level of breast cells,and its mechanism may be related to negatively regulated cells proliferation capacity directly,suggesting that miR-100 may be a potential effective target for prevention and treatment of breast cancer.