The relationship between the grain boundary steps on the specimen surface developed by cyclic bending and fatigue crack generation on the grain boundaries inclined at about 45 degree to the principal stress axis has been discussed for 4N-aluminum wire. The crystal lattice rotation near the grain boundary induced by slip bands in a grain resulted in a strain incompatibility with the neighboring grain. Due to the shear deformation along the grain boundary, the individual grain boundary steps coalesce and form a grain boundary microcrack, resulting in the fatigue crack generation and propagation. The strain incompatibility is developed at the grain boundary between the grain accompanied with crystal rotation over 10 degrees in the vicinity of the boundary and the grain with almost no crystal rotation. Based on the approximation that only the primary slip system is operative, the step height due to the crystal slip deformation is estimated from the lattice rotation near the grain boundary.