In this paper, fatigue strength behavior of ultraviolet irradiated high-modulus type poly-p-phenylene benzobisoxazole (PBO) fiber was investigated in monofilament tests. The tensile tests of a monofilament were carried out at a gauge length of 12.5 mm and deformation rate of 0.5 mm/min. The fatigue tests of a monofilament were carried out to determine the S-N property at a frequency of 10 Hz with three stress ratios of 0.1, 0.5 and 0.7. Irradiation time was set to 100 h, while radiance was arranged to become 8 W/m2. It was found that the tensile strength of 100 h-irradiated PBO fiber was represented by a two-parameter Weibull distribution. The tensile strength of 100 h-irradiated PBO fiber was lower than that of non-irradiated fiber. And, the fatigue strength of 100 h-irradiated PBO fiber was lower than that of non-irradiated fiber over all fatigue lives. The relationship between the maximum stress and fatigue life was independent of the stress ratio. In addition, it was found by SEM observation that 100 h-irradiated PBO fiber had split vertically in the surface to a depth of about 0.5μm, but non-irradiated fiber had not the split. Moreover, it was found by EDX analysis that the amount of carbon and oxygen at the surface layer of PBO fiber increased by irradiating UV ray. Thus, reduction in fatigue strength of the PBO fiber by UV irradiation was caused by a degradation of the fiber surface by autoxidation
