Optical fibers were irradiated with neutron fluxes from an atomic reactor. Characteristics of the irradiation loss with respect to transmission wavelengths were studied. We found that two types of absorption exist: that in the shorter wavelength and that in the longer wavelength. The absorption in the short wavelength, less than approximately 600 nm, increased with the increase in the irradiation time; however, the loss in the longer wavelength between approximately 600 and 900 nm increased with the irradiation time but saturated a few hours after the beginning of the irradiation. The difference is attributable to the fact that the absorption in the longer-wavelength range is related to the intrinsic defects in the optical fibers while the defects in the short-wavelength ranges are related to the breakdown of the SiO 2 network in optical fibers.