Objective　To investigate the radiosensitization of a new nucleoside analogue (E)-2'-deoxy-2'-(fluoromethylene) cytidine (FMdC) in vitro. Methods　Clonogenic assay was used to investigate the radiosensitizing effect of FMdC on a human colorectal cancer cell line WiDr and two cervical cancer cell lines C33-A and C4-I. The change in radiosensitivity was quantified by calculating the sensitization enhancement ratio at a clinically relevant dose of 2?Gy (SERSF2), defined as the mean survival fraction (SF) for control/mean SF for FMdC. Results　FMdC enhanced the radiosensitivity of WiDr cells in a time-course dependent manner. After treatment with 30?nmol/L FMdC for 6～72 h, no radiosensitization was observed at 6, 12 and 24 h, whereas an enhancement of radiosensitivity was seen at 48 h and 72 h. The corresponding SERSF2 was 1.33 and 1.79, respectively. With a high dose of 300?nmol/L FMdC exposed to WiDr cells for 6, 12, and 24 h, the SERSF2 were 1.19, 1.73 and 3.5, respectively. The radiosensitizing effect was dependent on the doses of FMdC. Pre-irradiation exposure of WiDr cells to 20～50?nmol/L FMdC for 48 h resulted in an increase of SERSF2 from 1.15 to 2.28. The observed radiosensitization was also confirmed on two human cervical cancer cell lines C33-A and C4-I with a SERSF2 of 1.32 and 2.08, respectively. Conclusions　These results show that FMdC is a potential radiosensitizer in colorectal cancer and cervical cancer cell lines.