Objectives To investigate the role of microRNAs in doxorubicin resistance of breast cancer cells. Results MicroRNA-134 was down-regulated in doxorubicin-resistant and doxorubicin-sensitive breast cancer samples and MCF-7/ADR cell lines. After transfection of miR-134 mimics, an MTT assay confirmed that the MCF-7/ADR cell proliferation was inhibited. In the presence of doxorubicin, there was inhibition of cell proliferation. Transfection of miR-134 mimics induced MCF-7/ADR cell apoptosis. The expression of ABCC1 was significantly upregulated in doxorubicin-resistant or -sensitive breast cancer tissues and MCF-7/ADR cell lines. Over-expression of miR-134 decreased the expression of ABCC1 at the protein level. Conclusion MicroRNA-134 modulates resistance to doxorubicin in breast cancer cells by downregulating the expression of ABCC1 which is known to encode the multidrug resistance-associated protein 1.