Abstract: Objective　To investigate the effect of miR-34a expression level on doxorubicin (DOX) resistance in breast cancer cells and its molecular mechanism. Methods　The expression levels of miR-34a in breast cancer cells (MCF-7) and DOX resistant cells (MCF-7/ADR) were detected by qPCR, and their expression levels were regulated by transfection of miR-34a mimics or miR-34a inhibitor. The half effective inhibitory concentration (IC50) of DOX treated cells was screened by CCK-8 method. CCK-8 method was used to detect cell survival rate. The percentage of apoptotic cells was detected by flow cytometry. ENCORI website predicted the binding site of miR-34a target gene Akt. The expressions of target gene protein kinase B (Akt), Bcl-2 and Bax proteins were detected by Western blot assay. Results　The expression of miR-34a in MCF-7/ADR was significantly lower than that in MCF-7 cells (P＜0.01). Compared with the NC inhibitor group, the expression level of miR-34a decreased in the miR-34a inhibitor group (P < 0.01). The values of IC50 of MCF-7 and MCF-7/ADR cells treated with DOX were 0.895 and 13.607 mg/L, respectively. There may be a binding site between miR-34a and the 3′UTR region of Akt gene. After DOX treatment, the survival rate increased significantly, the apoptosis rate decreased, and the levels of p-Akt/Akt and Bcl-2/Bax protein increased in the miR-34a inhibitor group (P＜0.05). Accordingly, compared with the NC mimics group, the expression level of miR-34a increased in the miR-34a mimics group (P＜0.01). After treatment with DOX, the cell survival rate decreased, the apoptosis rate increased, and the levels of p-Akt/Akt and Bcl-2/Bax protein decreased in the miR-34a mimics group (P＜0.01). Conclusion　The down-regulated expression of miR-34a in MCF-7/ADR cells may reduce cell apoptosis by stimulating Akt/Bcl-2 signal, thus enhancing the resistance of cells to DOX.