In order to investigate the influence of loading frequency on the fatigue strength and fracture mechanism of a high strength Al alloy, ultrasonic fatigue tests were carried out for an extruded Al alloy 7075-T 6 and the results were compared with those of rotating bending fatigue tests. Fatigue strength of ultrasonic was higher than that of rotating bending. The crack initiation and the crack growth rate in ultrasonic fatigue were delayed compared with those of rotating bending fatigue. In case of ultrasonic fatigue, the direction of crack growth changed from tensile mode to shear mode and many voids were observed on the fracture surface in shear mode growth, whereas the crack propagated in tensile mode and striations were observed in rotating bending fatigue.