The γ-ray radiolytic defect in optical fiber has been studied by the electron spin resonance method. It is found that a series of paramagnetic defects have been formed by the γ-ray irradiation; the densities of the defect centers increase with the irradiation doses, and the types of defects are mainly the E′ center, the non-bridging oxygen hole center and the perody radical (OHC). The defects production mechanism is also discussed.