This investigation deals with the mechanism of ductile crack initiation from a notch under mode II loading on a weldable structural steel SM41A. Under mode II loading, a notch root becomes acute and ductile crack initiation in shear type occurs at the region of the acute notch tip by large shear deformation. The ductile crack initiation occurs in two stages. In the first stage, the crack is initiated ahead of the acute notch tip by shear decohesion, and in the second stage, the crack emanates from the tension side of the surface of the first stage crack under low stress triaxiality. It seems to be difficult to predict analytically the shear type crack initiation from a notch under mode II loading, because of the intense concentration of strain at the acute notch tip and the complexity of stress history of material elements around notch tip. From the mode I loading tests of the specimens subjected to mode II preloading, it is found that the ductile crack initiation under mode I loading is promoted by mode II preloading. This result suggests that, at the notch tip region under mode II loading, some microscopic damage other than voids is produced, though unobservable by an optical microscope.