Previous investigations of frequency on fatigue strength are briefly reviewed in this paper. New resonant-type push-pull fatigue testing machines at 50 and 100 kc/s were also developed by using a ferrite magnetostrictive vibrator. By using these machines, as well as previously developed high and low-frequency push-pull fatigue testing machines, the influence of stress frequency on fatigue limit of two carbon steels was determined at frequencies ranging from 40 to 100,000 c/s. The fatigue limit was found to increase monotonically with increasing frequency. No peak in the value of the fatigue limit was observed at frequencies up to 100,000 c/s.