The object of this work is to study the radiation sensitizing effect and cytotoxicity to HepG2 cells of a newtapy bioreduction activity-5-(aziridin-1-yl)-3-hydroxymethyl-1-methylindole-4,7-dione(629) in vitro. Cytotoxicity of 629 or MMC in vitro were evaluated in HepG2 cells or hepatocyte by MMT method under anaerobic and aerobic, in vitro radiosensitizing ability was estimated using Cells Cloning Technology. The results suggested that the cytotoxicity of 629 to HepG2 cells has significant enhanced (,p,<,0.05) than MMC, and no significant cytotoxicity to hepatocyte; in addition, 629 has radiosensitizing effect in vitro on HepG2 cells and the SER value ranged from 1.17 to1.92, it is showed that 629 have powerful radiosensitizing effect than MMC. The results indicated that 629 has radiosensitizing characteristic with high efficiency and low toxicity, and it is promising for clinic applications.