The effect of the stress ratio (R = sigma(min)/sigma(max)) on the fatigue behavior of (+/- 45)(2s) angle-ply glass/epoxy composite laminates was investigated by comparing their mechanical, thermal, and optical properties under the stress ratio of 0.5 with previous fatigue results obtained under the stress ratio of 0.1. When the stress ratio was increased from 0.1 to 0.5, the fatigue life was enhanced at the same sigma(max) and the slope of S-N curve decreased, exhibiting more scattered responses. In addition, as the stress ratio increased, the fatigue damage was distributed more uniformly with a lower self-generated temperature at the same sigma(max). At R = 0.5, fiber realignment, due to cyclic creep, increased the fatigue stiffness, thus compensating the decreasing effect of fatigue damage. At high stress levels, the stiffening effect dominated the stiffness evolution, resulting in greater fatigue stiffness with an increasing number of fatigue cycles; however, at low stress levels the degrading effect due to fatigue damage was prevalent. The stiffening effect led to smaller hysteresis loop areas at the beginning of the fatigue experiments, which subsequently stabilized for the greater part of the specimen fatigue life, followed by a slight increase before failure. At R = 0.1, the fatigue stiffness decreased further and the hysteresis loop area became larger at all stress levels at the same sigma(max), since the stiffening effect was less and fatigue damage more severe.