In this paper we present dynamic fatigue of fused silica optical fibers. Time‐to‐failure of the fibers under repeated stress has been investigated theoretically and experimentally, and the results have been compared with the time‐to‐failure under constant stress and increasing stress at a constant rate. As a result, it is found that the time‐to‐failure under repeated stress is related to the maximum stress with the same parameternas those representing the times‐to‐failure by the other tests. The lifetime under repeated stress can be predicted by using two constants measured by the increasing test stress at a constant rate.