Although a number of models for the initiation of intergranular cracks under fatigue have been proposed, most of them are not considered to be sufficient qualitatively. In this study, the mechanism of intergranular crack initiation in fatigued copper is investigated on the basis of the observation of dislocation structures and surface topography by means of transmission and scanning electron microscopy. At the intersections of ladder-like structures with the grain boundary, extrusion-type intergranular deformations are frequently observed. It is also found that the ladder-like structures are sometimes formed along the grain and twin boundaries. On the basis of these observations, a mechanism of crack initiation at the grain boundary is proposed.