Abstract: Objective To establish the radioresistant human colon cancer cell line SW480-R，which could serve as a reference line for mechanistic studies of radioresistance. Methods Human colon SW480 cancer cells were repeatedly exposed to X-rays（2 Gy per fraction）.After exposing the cells for different periods， the novel radioresistant subline SW480-R was obtained.The doubling time of SW480 and SW480-R cells was determined by MTT assay，and relative radiosensitivity by standard colony formation assay.Survival rates after irradiation were measured by MTT assay. The distribution of cells in different stages of the cell cycle was investigated by flow cytometry before and after irradiation. Results Carcinoma cells survived after being exposed to 2 Gy for 3 weeks， and these cells were considered to be a radioresistant subline of SW480 cells.The resistant SW480-R cells showed greater radioresistance and longer doubling time than the parental SW480 cells（33.93 vs 24.87 h，P<0.05）.The resistant cells showed a higher survival rate after irradia-tion than did SW480 cells（98.40 vs 92.81%，P<0.001），and a higher percentage of cells in G2 /M phase. Conclusion The new subline SW480-R is more radioresistant than the parental SW480 cell line， and it has a different cell cycle distribution.

Key words:  Colon neoplasms, Radiotherapy, Radioresistant, Cell line