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In this paper, tensile strength and behavior of low-intensity UV light irradiated poly-p-phenylene benzobisoxazole (PBO) fiber were 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. Irradiation time was set to 0h, 1h, 10h, 100h and 1,000h, while radiance was arranged to become 2, 4 and 8 W/m2. It was found that the tensile strength distribution of UV irradiated PBO fibers can be approximated to a normal distribution. Regardless of the degree of radiance, the tensile strength tends to decrease gradually with an increase in irradiation time. As radiance intensifies, however, corresponding curved lines move to lower positions, an indication of the dependency of the tensile strength on radiance. The relationship between radiation dosage and tensile strength converges on this one curved line irrespective of the degree of radiance. Therefore radiation dosage should be a valid parameter to measure the degradation of the strength of the PBO fibers exposed to UV light irradiation. In addition, it is found by SEM observation that there are distinct differences between the fracture surface image of UV non-irradiated fiber and that of irradiated fiber. Regardless of UV-irradiation, PBO fibers have split in the direction in which it is set. But the split part in UV-irradiated fiber is shorter than in the UV-non-irradiated fiber because UV-irradiated fiber has split vertically in portions.