Synchrotron X-ray microtomography has been used for the in-situ observation of three-dimensional (3D) fatigue crack propagation of an aluminum alloy. Local crack growth rates were calculated at different positions along specimen thickness. Crack observation revealed that crack propagation was frequently retarded in local region where crack overlapped crack segments exist. Propagation of cracks into overlapped region was strongly constrained due to stress shielding. Observation of crack propagation process indicated that the overlapped crack segments generally corresponds to the potion of twisted crack. Crack tip opening displacement was measured and it was found that crack opening was larger in single planar crack than that in crack overlapping region.