Malignant neoplasms are often refractory to radiotherapy because they contain areas of hypoxic cells that tolerate irradiation, reducing the effect of the treatment. If these areas of hypoxic cells can be oxygenated, the effect of radiotherapy is expected to be enhanced. Hyperbaric oxygen therapy was devised in the 1950s, and the radiosensitizing agent Misonidazole was developed in 1970. However, neither produced satisfactory clinical effects in radiotherapy of tumors. In this study, hypoxic cells in a solid tumor were efficiently oxygenated by the use of perfluorochemicals (PFC) developed as artificial blood with carbogen gas (CG), and the anti-tumor effect of irradiation was enhanced. In C3H mice bearing RIF-1 tumor, the mean oxygen pressure increased to 79.8 mmHg in those treated with PFC and CG as compared with 12.9 mmHg in the controls, and the does modification factor in irradiation of these mice was TCD50 1.47. PFC is currently under clinical trials, and we also noted effective oxygenation of tumors. These findings indicate the usefulness of PFC as a radiosensitizing agent.