The effect of torsional mean stress on the fatigue behavior of glass fiber-reinforced polyester (GFRP) is studied by testing thin-walled, woven-roving tubular specimens with two fiber orientations, and , at negative stress ratios , 0. The specimens were found to have higher fatigue strength than the specimens at all stress ratios. This is attributed to the difference in local stress components, the specimens being subjected to tension-compression local stress components, while the specimens being subjected to pure local shear stress. For the studied stress ratios; the mean stress component had a detrimental effect on the amplitude component for the specimens; while it was ineffective for the specimens in a certain region in the mean-amplitude diagram, region (1), then it had a detrimental effect in the rest of the diagram, region (2). The curves for positive stress ratios were extrapolated from those for negative stress ratios, which were found experimentally, for the specimens. The positive stress ratio points, having the same local stress state as the negative ones, showed an acceptable behavior tending to decrease the amplitude component for the same life.