Fatigue crack growth (fcg) and threshold behavior of engineering alloys may be studied using an ultrasonic fatigue device, in which the loading frequency is 20 kHz. In this investigation, the stress intensity factors in ultrasonic fatigue testing were determined through dynamic modal analysis and three-dimensional finite element calculation, which, to our knowledge, has never been achieved before in ultrasonic fcg studies. For a better comprehension, the operating principle of the ultrasonic fatigue machine, specimen design as well as experimental procedures, was briefly presented.