[Abstract] Objective: To explore the influence of miR-339-5p on the radio-sensitivity of lung cancer A549 cells by regulating the expression of Nudix hydrolase 5 (NUDT5). Methods: X-ray-resistant lung cancer A549 cells (RA549) were induced by treatment with low concentration gradient increment combined with large dose intermittent shock in vitro. The expression level of miR-339-5p in hu‐ man normal lung epithelial cells (BEAS-2B) and lung cancer cell lines (A549, L78, H1299, H460 and RA549 cells) was detected by qP‐ CR. According to the treatment, RA549 cells were divided into NC group, 5Gy group (treatment with 5Gy X-ray), 5Gy+miR-339-5p mimic group, 5Gy+si-NUDT5 group and 5Gy+si-NUDT5+miR-339-5p inhibitor group. CCK-8 assay, Annexin V-FITC/PI double stain‐ ing flow cytometry and WB were used to detect the proliferation, apoptosis and the protein expressions of NUDT5, γ-H2AX and H2AX in each group. The targeting relationship between mir-339-5p and NUDT5 was detected by Dual-luciferase reporter gene system. Results: The expression of miR-339-5p in lung cancer cell lines was significantly lower than that in BEAS-2B cells, with the lowest ex‐pression level in RA549 cells (all P<0.05). NUDT5 was the target gene of miR-339-5p. Compared with the NC group, the prolifera‐ tion activity and NUDT5 expression of RA549 cells in the 5 Gy group were significantly reduced (all P<0.01), and the apoptosis rate was significantly increased (P<0.01). Compared with the 5 Gy group, the proliferation activity of RA549 cells in the 5 Gy+miR-339-5p mimic group was significantly reduced (P<0.05), the apoptosis rate ([12.97±1.48]% vs [5.21±0.62]%, P<0.01) and the expression level of γ-H2AX (P<0.05) were significantly increased; the expression of NUDT5 (t=7.58, P<0.01) and cell proliferation activity (t=6.58, P<0.01) of RA549 cells in the 5 Gy+si-NUDT5 group were significantly reduced, while the apoptosis rate ([11.21±1.06]% vs [5.54±0.44%, P<0.01) and the expression of γ-H2AX (P<0.01) were significantly increased; and the above indicators in 5 Gy+si-NUDT5+miR-339-5p inhibitor group showed insignificant difference from the 5 Gy group. Conclusion: Overexpression of miR-339-5p enhances the radio-sensitivity of X-ray-resistant lung cancer A549 cells by targetedly down-regulating NUDT5 expression.