Objective To explore the mechanism of microRNA-145 (miR-145) involved in proliferation and apoptosis of breast cancer MCF-7 cells.  Methods The immortalized breast cancer cell line MCF-7 cells were cultured in vitro and transfected with miR-145. Real-time PCR and Western blotting were used to detect mRNA and protein levels, respectively. The proliferation level of each group was detected by cell counting kit-8 (CCK-8) method , and the apoptosis level of MCF-7 cells in different treatment groups was detected by flow cytometer.  Results Real-time PCR result showed that miR-145 did not affect Caspase-3, proliferating cell nuclear antigen (PCNA) and B cell lymphoma factor 2 (Bcl-2) mRNA levels in MCF-7 cells. Western blotting analysis showed that compared with the control group, transfection of miR-145 for 96 hours significantly increased the expression of Caspase-3 and inhibited the expression of PCNA and Bcl-2. The result of CCK-8 assay showed that the proliferation rate of MCF-7 cells was decreased after overexpression of miR-145 for 72 hours and 96 hours (P<0.05). The result  of flow cytometer showed that the apoptosis rate of MCF-7 cells in overexpression group was significantly higher than that in the control group (P<0.05).  Conclusion MiR-145 can inhibit the proliferation and promote the apoptosis of MCF-7 cells by down-regulating PCNA and Bcl-2 and up-regulating the expression of Caspase-3, which may be a new target for breast cancer treatment.