Influences of stress ratio on the off-axis fatigue strength of a unidirectional fiber-reinforced composite T800H/2500 have been studied. Tension-tension and tension-compression fatigue tests on plain coupon specimens with fiber orientation angles θ = 0, 10, 15, 30, 45 and 90° were performed for stress ratios R = 0.5, 0.1, -0.3 (θ= 0°) and R = 0.5, 0.1, -1.0 (0°< θ ⪇ 90°), respectively. Also, a fatigue damage mechanics model that considers the effect of stress ratio was formulated. Fatigue strength of the unidirectional T800H/2500 decreased as the stress ratio decreased, irrespective of off-axis angle. The S-N relationships were almost linear in the range of fatigue loading up to 106 cycles, regardless of off-axis angle and stress ratio. The non-dimensional effective stress based on the classical static failure theory succeeded in a unified description of the directional characteristic of the off-axis fatigue strength for all stress ratios. The proposed fatigue damage mechanics model can predict the stress-ratio dependent off-axis fatigue behavior of the unidirectional system employed.