A testing equipment and measurements are reported of the fatigue of metals under supersonic frequency. This equipment has an advantage over conventional fatigue machines that stress alternations up to 10⁸ reversals can be made within an hour or so. At the resonant condition, metal samples are subjected to alternating stresses above the fatigue limit by means of a magnetostriction transducer through an exponential brass horn. By this method the fatigue properties of various metals were investigated. To avoid temperature rise of the samples, water cooling was necessary. Under insufficient cooling, it was not possible to obtain large motions and high strains, presumably due to the increase of internal damping in the samples. Even marks in the most strained part of sample such as might be made by a very light scratch with a knife edge had a considerable effect on the fatigue limit.