Abstract: Fatigue crack growth (FCG) and threshold behavior of four widely used engineering alloys have been investigated at high frequency Z(f =20kHz) by means of an ultrasonic fatigue device.Specimen design has been given.Crack propagating rate da/dN was obtained between 10^-9 and 10^-5 mm/cycle in function of stress intensity factor △K ,which was determined by a modified analytic experimental method.For comparison,conventional FCG data ( f =2 to 50Hz) were carried out with a high stress ratio (R=0.7) applied.High frequency effect on the fracture behavior of different materials was discussed and the failure mechanism was obtained in the two different regimcs.