Abstract: The fatigue strength of composite materials was first investigated under cyclic tension of plastics reinforced by fiberglass. Since that time, intense investigations of the fatigue strength of composite materials have begun. The experimental data obtained for different types of composite materials show that the fatigue-fracture processes are induced by the accumulation of defects of various nature. The main mechanisms for determining the durability of composites are the kinetics of development of the damaged state until final fracture. At the same time, according to the experimental results, the total-damage-accumulation curve is an increased function of time (the number of loading cycles) until the moment of macrofracture. In this study, on the basis of these investigations and the concept of scattered damage and fracture, the fatigue strength criterion is formulated. The fatigue criterion coefficients are specified, and the damage-accumulation curves are plotted in dependence on the number of cycles and the stress level. A comparison with the experimental results on the fatigue of glass-reinforced plastics and carbon-filled plastics is given.