Abstract A phenomenological fatigue model is developed for the off-axis fatigue behavior of a class of unidirectional polymer matrix composites subjected to constant-amplitude stress cycling with non-negative mean stresses. For this purpose, a new non-dimensional effective stress that considers the effects of the stress ratio and multiaxial state of stress on the off-axis fatigue behavior of unidirectional composites is defined for the range of non-negative mean stresses. Non-dimensional effective stress is used as the strength measure to characterize the brittle nature of the evolution of fatigue damage. This allows one to obtain a practical form of multiaxial fatigue model that takes into account the fiber-orientation dependence as well as the mean-stress dependence of the off-axis fatigue behavior of unidirectional composites. The constant-fatigue life diagram predicted by the proposed fatigue model is described in terms of a normalized relation of the modified Goodman type. It is demonstrated that the off-axis S– N relationships for unidirectional carbon/epoxy and glass/epoxy composites are adequately described using the proposed fatigue model over the range of non-negative mean stresses.