The DNA damage response (DDR) encompasses multi-step processes by which cells evolve to sense DNA damage, transduce the signal and initiate the repair of damaged DNA. Ataxia Telangiectasia Mutated (ATM) Kinase, which functions as the primary sensor and transducer of DNA damage signal, has been demonstrated to play an important role in the DDR and cancer prevention. Hence, understanding the molecular mechanisms underlying the regulation of ATM has received much attention. Here, we found that miR-18a was upregulated in both cell lines and patients' tissue samples of breast cancer. Furthermore, we demonstrated that ectopically expressing miR-18a downregulated ATM expression by directly targeting the ATM-3'-UTR and abrogated the IR-induced cell cycle arrest. Similar to the effect of ATM siRNA, overexpressing miR-18a in breast cancer cells reduced the DNA damage repair ability and the efficiency of homologous recombination-based DNA repair (HRR) and sensitized cells to γ-irradiation (IR) treatment. However, inhibition of miR-18a led to augmentation of DNA damage repair, increase of HRR efficiency and reduced cellular radiosensitivity. Moreover, we showed that the phorsphorylation level and nuclear foci formation of H2AX and 53BP1, the downstream substrates of ATM kinase, were significantly deceased in miR-18a overexpressing cells. Taken together, our results uncover a new regulatory mechanism of ATM expression and suggest that miR-18a might be a novel therapeutic target.