Non‑small cell lung cancer (NSCLC) is radioresistant to X‑rays due to powerful cellular DNA damage repair mechanisms. DNA‑dependent protein kinase (DNA‑PK) is a key enzyme involved in DNA damage repair and the phenomenon and molecular mechanism of NSCLC radionsensitivity were investigated following inhibition of DNA‑PK activity. In the present study A549 cells were treated with the DNA‑PK inhibitor NU7026 and/or siRNA directed against ataxia telangiectasia mutated (ATM), followed by exposure to 4 Gy X‑ray irradiation. Radiosensitivity, DNA damage, apoptosis and protein expression were measured by colony formation assay, γH2AX foci immunofluorescence, Annexin V/PI staining and western blotting, respectively. A Balb/c‑nu/nu xenograft mouse model was established by subcutaneous injection of A549 cells and was used to examine the effect of administering NU7026 via intraperitoneal injection prior to 4 Gy X‑ray exposure. The xenograft tumors were weighed and observed by hematoxylin and eosin staining after irradiation. NU7026 treatment followed by X‑ray irradiation significantly decreased the colony formation ratio of A549 cells, and increased γH2AX foci and cell apoptosis. Furthermore, the combined treatment of NU7026 and X‑rays resulted in growth inhibition and cell apoptosis in A549 xenograft tumors. Consequently, apoptosis regulators full‑length transactivating (TA) p73 and an N‑terminally truncated (DN) p73 were upregulated and downregulated respectively, leading to activation of glucosyltransferases and Rab‑like GTPase activators and myotubularins domain‑containing 4 (GRAMD4) protein to reduce the Bcl‑2/Bax protein ratio. In addition, ATM siRNA efficiently prevented γH2AX foci formation, and enhanced NU7026‑induced inhibition of survival and promoted apoptosis. In conclusion, inhibition of DNA‑PK activity increased the radiosensitivity of A549 cells to X‑ray irradiation. NU7026 treatment activated the ATM‑dependent DNA damage response and induced p73 apoptosis pathway. DNA‑PK inhibitor may be an effective constituent of radiosensitization products. DNA damage repair pathway could be a potential target for radiosensitization.