A nitroimidazole with hypoxic selectivity and radiosensitizing property. Pimonidazole is reduced in hypoxic environments as in tumor cells, thereby it can be used as an hypoxia marker. In hypoxic cells, reduced pimonidazole binds to -SH-containing molecules such as glutathione and proteins, and the resulting complexes accumulated in tissues, thereby sensitizing cells to be more susceptible for radiation treatment.