It is known that when buckling collapse takes place in structural members, a crack may easily initiate at the concave side of buckling deflection. This is due to the deteriorating effect of large compressive prestrain on the ductility of steels. In this paper, an attempt was made to quantify the crack initiation strength for structural steel members having experienced large compressive prestrain. Two kinds of experiments, a tensile test for prestrained material and a crack initiation test for strip specimens, were performed on JIS SS 400 steel. In the former tests, the influence of prestraining on the mechanical properties of material was investigated, while in the latter the specimens were bent into various curvatures after undergoing buckling and the relationship between crack initiation and applied straining was examined. Elastoplastic large deformation FEM analyses were also performed on the strip specimens to investigate stress and strain distributions at the highly strained cross section. It has been found that : (1) With the increase in compressive prestrain, elongation of material is decreased, while ultimate tensile strength is increased. True fracture stress shows smaller dependency on the magnitude of prestrain. (2) For the strip specimens subjected to more than 4055%of compressive strain at the concave side of bending, initiation of surface hair cracks was observed when unloaded. (3) According to the FEM analyses, large tensile residual stress is induced at the concave side of bending when unloaded, and this is considered to be a primary cause of crack initiation. (4) It is possible to assess the crack initiation by comparing the hydrostatic component of residual stress with ultimate tensile strength and/or true fracture stress of prestrained material.