Abstract: In order to search for effective radiosensitizer of hypoxic cell with low toxicity,a series of 5-bromo-,5-methy1-,and 5-unsubstituted-3-nitro-1,2,4-triazole-1- acetamides were synthesized and tested in vitro with HeLa S3 cells.The radiosensitlzing activity of 5-bromo-derivatives are more potent than those of corresponding 5-methyl-or 5- unsubstituted-1,2,4-triazole derivatives,but the chronic aerobic cytotoxicities are increased.Modification of the N¹-acetamide side chain can also change the radiosensitizing activity and the lipophilicity.Among all the tested nitrotriazole derivatives,TA-1012-(3- nitro-1-triazolyl)acetamide,may be a potential radiosensitizer because of its higher sensitizing activity and much lower lipophilicity.