Abstract <p>The effect of mean stress on the fatigue strength of a polyphthalamide composite with 33 wt % short glass fibers was studied under various loading conditions, including cyclic tension, tension–compression, and compression. The mean stresses were 45, 0, –60, and –120 MPa, and the stress amplitudes ranged from 15 to 80 MPa. The results were compared with the known models by Goodman, Soderberg, and Gerber, highlighting the ambiguity in predicting the effect of mean stress in the region of high positive and negative mean stress values. At negative mean stresses, the limiting stress amplitudes were higher, when the fatigue resistance increased until the material reached its compressive yield stress, after which a decrease was observed again. Differences in the fatigue damage mechanisms were identified in cyclic tension and cyclic compression modes. In the tension mode, fatigue damage developed at the fiber–matrix interface due to adhesive fracture. In contrast, in cyclic compression mode, the main mechanism of fatigue damage accumulation was creep followed by cohesive fracture of the matrix.</p>