Objective:To explore the effect of miR-342-3p on chemotherapy sensitivity of breast cancer cells. Methods:The expression levels of miR-342-3p were detected in breast cancer cell lines MCF-7,SKBr3 and MDA-MB-231. By using lipofectamine,the hsa-miR-342-3p mimic was transfected into breast cancer cell lines,which were of the lowest expression of miR-342-3p cell lines (the mimic group). The mim-NC was performed as the negative control group. Furthermore,the miR-342-3p inhibitor was transfected into breast cancer cell lines of the highest expression of miR-342-3p,the inhi-NC was performed as the negative control group. Cells from four different groups (the mimic,mim-NC,inhibitor and inhi-NC groups) were treated with 2 -滋mol/L paclitaxel,2 -滋mol/L cisplatin and 4 -滋mol/L doxorubicine for 48 hours,respectively. CCK8 assay was used for detection of cell proliferation. Results:Compared with the expression level of miR-243-3p in SKBr3,the level of miR-243-3p in MCF-7 cells was significantly increased (126.000 fold change),but it was decreased (0.017 fold change) in MDA-MB-231 cell lines. The rates of cell proliferation in the mimic group after treatment with paclitaxel and cisplatin for 48 hours were significantly lower than those in the mim-NC group,respectively (P < 0.05,respectively). However,the cell proliferation rates in the mimic group and the mim-NC group after treatment with doxorubicine for 48 hours had no significant difference (P > 0.05). The cell proliferation rates in the inhibitor group were significantly higher than those in the inhi-NC groups after treatment with paclitaxel,cisplatin and doxorubicine for 48 hours,respectively (P < 0.05,respectively). Conclusion:miR-342-3p may play a key role in the regulation of chemotherapy sensitivity to paclitaxel and cisplatin in breast cancer cell lines MDA-MB-231 and MCF-7. Up-regulation of miR-342-3p expression could not increase the chemotherapy sensitivity to doxorubicine,but down-regulation of miR-342-3p expression may weaken the chemotherapy sensitivity to doxorubicine.