This study has compared radiosensitization effects of metronidazole derivative (CM) after 8, 24 and 32 Gy r-ray irradiation in S,180, bearing mice as a model of tumor. After comparing the ratio of Vt/Vo, the inhibition rate of tumor volume, the tumor grwth delay, the treatment efficiency, ratio of the area of growth curve and the average survival time, the results showed that CM has more effective radiosensitization effects at lower does level (8 Gy)than at higher dose levels (24, 32 Gy). In the latter case, there was no obvious statistical difference.