In this paper, tensile and fatigue properties of poly-p-Phenylene benzobisoxazole (PBO) fiber have been investigated. The tensile test was carried out with a gauge length of 12.5 mm and displacement rate of 0.5 mm/min. The tensile strength data were analyzed in terms of the Weibull probability distribution function. Fatigue test was performed using an electromagnetic force fatigue tester. The fatigue testes were carried out at a frequency of 10 Hz with three stress ratios of 0.1, 0.5, and 0.7. It was found from the tensile strength test that the distribution of PBO fiber strengths expressed as Weibull distribution function with 2 parameters. The test results clearly showed that the fatigue behavior appeared in the S-N characteristic curves. The relation between the stress amplitude and the fatigue life depended on the stress ratio. However, the relation between the maximum stress and the fatigue life was independent of the stress ratio, and the maximum stress was most suitable parameter to describe the fatigue strength at the different stress ratios. Consequently, it became clear that the fatigue strength of the PBO fibers was not cycle dependent, but is time dependent. In addition, it is found by SEM observation that there are distinct differences between the surface image of tensile test specimen and that of fatigue test specimen, i.e., the crack length in the fatigue test specimens are much longer than that in the tensile test specimens.