The phenomenon of ductile blunting under plane stress conditions in cracked polycarbonate plates was studied. Because this phenomenon is intimately connected with the amount of crack opening displacement (CTOD) and its analogous phenomenon of crack tip advance displacement (CTAD), a study was undertaken of the mechanism of the development of blunting by evaluating the mode of evolution of CTOD and CTAD in the specimens. As study by scanning electron microscopy is limited to a thin layer of the surface of the specimen, this method is most convenient for studying blunting phenomena under plane-stress conditions. Interesting results were derived from these experiments and the characteristic properties of plane-stress blunting were determined.