BACKGROUND: MicroR-10 b can regulate the characteristics of breast cancer stem cells, and acetaldehyde dehydrogenase 1 (ALDH1) is one of the most important markers of breast cancer stem cells. The interaction between them in breast cancer cells needs further explorations. OBJECTIVE: To investigate whether over-expression of microRNA-10 b affects ALDH1 mRNA and protein levels in human breast cancer MCF-7 cells. METHODS: hsa-miR-10 b mimics or its negative control was transfected into breast cancer MCF-7 cell line. At 48 hours after transfection, the mRNA and protein expression of ALDH1 in the cells was detected using real-time RT-PCR and western blot assays, respectively. RESULTS AND CONCLUSION: Over-expression of microR-10 b was found in the MCF-7 cell line transfected with hsa-miR-10 b mimics, which was significantly higher than that in the control group (P=0.003 47). Both of ALDH1 mRNA and protein levels were up-regulated in the MCF-7 cell line overexpressing microR-10 b, as compared with the control group (P=0.009 54 and P=0.003 11, respectively). To conclude, over-expression of microR-10 b induces the ALDH1 mRNA and protein expression in the breast cancer MCF-7 cell line, providing new evidence that microR-10 b regulates the invasion and metastasis of breast cancer cells.