Abstract: OBJECTIVE To investigate the role of miR-30a-5p in human breast cancer MCF-7 cells against tamoxifen(TAM) resistance and to clarify the relevant mechanism. METHODS MTT assay was used to detect the changes of drug resistance in MCF-7 cells induced by short time and high concentration of TAM stimulation. The expression of miR-30a-5p in MCF-7 cells and its drug-resistant cell line MCF-7/TAM was detected by qRT-PCR. The changes of autophagy in MCF-7/TAM cells were detected by acridine orange staining and Western blotting. After transfection with miR-30a-5p mimic, the sensitivity of MCF-7/TAM cells to TAM was detected by MTT assay, and the effect of miR-30a-5p on autophagy of MCF-7/TAM cells was observed by acridine orange staining and Western blotting. Bioinformatics method was used to predict the target gene of miR-30a-5p, and luciferase reporter assay verified the targeted regulatory effect of miR-30a-5p on ATG5. The effect of miR-30a-5p on the expression of ATG5 were detected by Western blotting. RESULTS The inhibition of MCF-7/TAM cells to TAM was significantly decreased than that of MCF-7 cells, The expression of miR-30a-5p in MCF-7/TAM cells was significantly lower than that in MCF-7 cells. Compared with MCF-7 cells, the autophagy level of MCF-7/TAM cells was significantly increased. After overexpression of miR-30a-5p, the sensitivity of MCF-7/TAM cells to TAM was significantly increased, and the autophagy level was significantly reduced. Targetscan analysis showed that ATG5 was the downstream target gene of miR-30a-5p, and luciferase reporter gene experiment further proved that miR-30a-5p targeted regulation of ATG5, and up-regulated miR-30a-5p could inhibit the expression of ATG5. CONCLUSION miR-30a-5p targetedly regulates ATG5 and inhibits autophagy, thereby enhancing the sensitivity of breast cancer cells to TAM.