The object of this work is to study the radiation sensitizing effect of suifonamide imizole in vitro and in vivo. In vitro radiosensitizing ability was estimated using V79 cells under hypoxia condition. Results show that SIM has radiosensitizing effect in vitro on Chinese hamster V79 cells and the SER value ranged from 1.12 to 1.81. In vivo radiosensitizing ability was estimated using C57BL mice bearing Lewis lung cancer. Different concentrations of drug were administered i.p. and local irradiation of 10Gy with ,137,Csγ-ray was given at 2h after administration. Tumor growth was observed for 20days and the tumor volume was measured every 2days. Results show that the tumor growth was suppressed after the treatment in the irradiated groups with SIM. The study indicates that SIM is a novel radiosensitizer.