The fatigue behavior of optical glass fibers was determined in air at 23°C and 55&percent; relative humidity by the dynamic‐fatigue test technique in which strength is measured as a function of stressing rate. The good correlation found between the fatigue test data and fracture‐mechanics theory indicates that failure is controlled by slow crack growth of preexisting flaws and that fracture‐mechanics theory can be used in making failure predictions for optical glass fibers.