To investigate the effects of loading frequency on fatigue crack growth behavior, ultrasonic fatigue tests were carried out for an extruded age-hardened Al alloy 7075-T6 in ambient air and in N2 gas. The results obtained were compared with those in rotating bending fatigue. Fatigue strength was higher in ultrasonic fatigue than in rotating bending fatigue. This may be caused by the retardation of crack initiation and early crack propagation. In ultrasonic fatigue, the growth direction of cracks changed macroscopically from tensile mode to shear mode with the direction oblique about 35 degrees to the stress axis. Fracture mechanism involved changed from striations featured to transgranular facets and microvoids predominated. Crack growth rate at growth mode transition was ~3x10-9 m/cycle. The relation between applied stress, a, and crack depth, bT, at the transition sites can be approximated as anbT = constant. The results were discussed from viewpoints of crack tip plasticity, time dependent environment effect and the texture microstructure of the alloy.