Cancer patients often suffer from local tumor recurrence after radiation therapy.Cell cycle, an intricate sequence of events which guarantee the high genomic fidelity, has been suggested to affect DNA damage responses and eventual radioresistant characteristics of cancer cells.Here, we established a radioresistant lung cancer cell line, A549R, by exposing the parental A549 cells to repeated γ-ray irradiation of a total dose of 60 Gy.The radiosensitivity of A549 and A549R was confirmed using colony formation assay.We then especially examined the cell cycle distribution between A549 and A549R and found that compared to parental A549 cells, A549R cells showed a statistically significant decrease in G1 phase and increase in S and G2/M phase.When A549 and A549R cells were exposed to 4 Gy irradiation and the cell cycle analysis showed a significant increase of G2-M phase cells and it suggested that G2-M cell cycle arrest plays a predominant role in mediating the radioresistance.In order to explore the possible mechanism behind the cell cycle related radioresistance, we examined the expression of Cdc25 proteins which orchestrate in the cell cycle transition.The results showed that Cdc25c, which plays a important role in G2-M arrest, determines the radioresistance of cells.