Purpose: To determine the biological effectiveness of single, fractionated and continuous low dose rate irradiation on the human colorectal cancer cell line CL187 in vitro and explore the cellular mechanisms.Materials and methods: The CL187 cells were exposed to radiation of 6 MV X-ray at a high dose rate of 4Gy/min and 125I seed at a low dose rate of 2.77 cGy/h.Three groups were employed: single dose radiation group (SDR), fractionated dose radiation group (FDR) by 2Gy/f and continuous low dose rate radiation group (CLDR).Four radiation doses 2, 4, 6 and 8Gy were chosen and cells without irradiation as the control.The responses of CL187 cells to distinct modes of radiation were evaluated by the colony-forming assay, cell cycle progression as well as apoptosis analysis.In addition, we detected the expression patterns of DNA-PKcs, Ku70 and Ku80 by Western blotting.Results: The relative biological effect for 125I seeds compared with 6 MV X-ray was 1.42.48 hrs after 4Gy irradiation, the difference between proportions of cells at G2/M phase of SDR and CLDR groups were statistically significant (p =0.026), so as the FDR and CLDR groups (p =0.005).48 hrs after 4Gy irradiation, the early apoptotic rate of CLDR group was remarkably higher than SDR and FDR groups (CLDR vs.SDR, p =0.001; CLDR vs.FDR, p=0.02), whereas the late apoptotic rate of CLDR group increased significantly compared with SDR and FDR group (CLDR vs.SDR, p =0.004; CLDR vs.FDR, p =0.007).Moreover, DNA-PKcs and Ku70 expression levels in CLDR-treated cells decreased compared with SDR and FDR groups.Conclusions: Compared with the X-ray high dose rate irradiation, 125I seeds CLDR showed more effective induction of cell apoptosis and G2/M cell cycle arrest.Furthermore, 125I seeds CLDR could impair the DNA repair capability by down-regulating DNA-PKcs and Ku70 expression.