Abstract: Stress and modal analyses of an ultrasonic vibration system,consisting of a notched specimen and one or two amplifying horns have been performed by using 3D finite element calculation.The stress intensity factors in ultrasonic fatigue crack propagation are evaluated by means of displacement and energy approaches.The particular advantages as well as limitations of the two approaches are briefly discussed.Two types of ultrasonic fatiuge loading with different stress ratio are exerted on the specimen respectively.The comparison between these two approaches comes to the conclusion that the energy approach is more accurate.It has wide range of practicality in engineering.