The high cycle fatigue of 6156 aluminum alloy was investigated under the conditions of different orientations and stress concentration factor. In addition, the microstructures and the fatigue fracture surfaces of the alloy were examined by optical, transmission and scanning electron microscopy. The results show that the effect of different orientations on the fatigue life of 6156 aluminum alloy is rather little and the fatigue life of Kt=1 is longer than that of Kt = 3, exhibiting notch sensitivity. The second phase particles play a key role during fatigue crack initiation, and most part of the cracks originate from the coarse particles on the surface or from the interface between the second phase particles and matrix.