Abstract: General mechanisms for effects of -ray radiation on optical fibers were studied. Absorption coefficient of -ray, cross sections of different effects, energy and flux and angle distributions of resulting Compton recoil electrons in optical fibers were calculated. An experimental method for transient radiation-induced loss measurement was presented. In order to measure transient radiation loss induced by pulsed -ray in optical fibers, it was developed that an experimental measurement system which applied analog broad bandwidth optical fiber links with five different wavelength lasers such as 405, 660, 850, 1 310 and 1 550 nm. Two different kinds of pulsed -ray devices with average photon energy of 0.3 MeV, dose rate of 2.03107 Gy/s and average photon energy of 1.0 MeV, dose rate of 5.32109 Gy/s were used to irradiate four types of optical fibers in experiments. The transient radiation-induced loss and its relationship with total doses of exposure were measured, and also the permanent radiation-induced losses with light spectrum, changes of refractive index. The experimental results show that: (1) The transient radiation-induced loss will increase as the detecting laser wavelength shifts from near-infrared to visible regions of optical spectrum.(2) Under the same experimental condition, the transient radiation-induced loss of multimode fibers is slightly larger than single-mode fibers.(3) The radiation of -ray will slightly decrease the refractive index of optical fibers.(4) Within a certain dose range transient radiation-induced loss in multi-mode fiber displays a nearly linear dependence upon total dose. The conclusion can be deduced that the generation of new color centers in fiber materials due to Compton electrons will increase the absorption loss and the changes of refractive index will lead to additional waveguide loss in optical fibers. Both radiation-induced loss mechanisms exist simultaneously; therefore, radiation-induced loss is the result of joint action of the two.