In this paper, fatigue strength of high-modulus (HM) type poly-p-phenylene benzobisoxazole (PBO) fiber improved tensile modules by heat-treatment have been investigate. To reveal the fatigue strength of fiber with different diameters, it is necessary to take into account the size effects on it in the diameter direction. In this report, such effect in the diameter direction is cut off from the experimental fatigue strength so as to evaluate is closely. As a result, variance in date on HM fiber stress ratios is small, and fatigue strength can be approximated to straight lines falling down from left to right on a single logarithm graph. Fatigue strength on stress ratio R=0.1 becomes lower than those of R=0.5, 0.7 and 1.0.
