Abstract: Objective To investigate the expression of miRNA-34a in human breast cancer tissues and cell lines and its effects on the proliferation, invasion, metastasis, apoptosis and cell cycle of MDA-MB-231 cell line. This research provides theoretical basis for the deep understanding of the role which miR-34a plays in breast cancer occurrence and development. Methods The different expression of miR-34a in 20 cases of human breast cancer tissues and normal tissues, as well as in the breast cancer cell line MDAMB-231, MCF-7 and normal breast epithelial cell line MCF-10A were detected by real-time PCR. Applying liposome transfect technique, miR-34a??mimic and negative control were transfected into MDA-MB-231 respectively,and the changes of cell proliferation, invasion, migration, apoptosis and cell cycle were analyzed. Results The expression of miRNA-34a was decreased in human breast cancer tissues(P＜0.01), while increased in MDA-MB-231, MCF-7 and MCF-10A cell lines successively(P＜0.01).Compared with MDAMB-231 cell transfected with miR-NC, the ability of proliferation, invasion and migration of MDA-MB-231 cell decreased after the transfection of miR-34a mimic (P＜0.01), the cell apoptosis increased(P＜0.01), and cell cycle was arrested in the G1/G0 phase(P＜0.01). Conclusion The expression of miRNA-34a both in breast cancer tissues and cell lines are signifi cant lower than that in normal tissues and MCF-10A. miR-34a can inhibit the proliferation, invasion and migration of MDA-MB-231, increase the rate of cell apoptosis, and arrest cell cycle in the G0/G1. miR-34a may act a tumor suppressor and its expression level is closely related to the occurrence and progression of breast cancer.