To obtain real-time dose information in photon-beam therapy using a clinical linear accelerator, we fabricated a novel Cerenkov fiber-optic dosimeter using two plastic optical fibers without employing a scintillator. In this study, the light intensity and spectrum of Cerenkov radiation induced by a high-energy photon beam were measured as functions of the irradiation angle and the length difference between the two plastic optical fibers in the dosimeter probe. Also, we obtained a percentage depth dose curve for a 6 MV photon beam with a field size of 10 × 10 cm(2) according to the depth of the solid water phantom. Based on the results of this study, it is anticipated that the proposed Cerenkov fiber-optic dosimeter can be developed as a useful dosimeter to accurately obtain dose information prior to conducting radiotherapy.