A bilayer specimen composed of a thin polystyrene (PS) film on a polycarbonate (PC) mini-bar was prepared for tensile test. It was observed by scanning electron microscopy that periodic cracks appeared in the PS film when the specimen was deformed in tension. From the maximum crack spacing in the PS, the ultimate shear stress at the PS/PC interface was estimated to be 49 MPa using the periodic cracking method. Based on the deformation mechanism operative in the glassy polymer and the measured crack spacing in the PS film, a new methodology was proposed and applied to predict the critical particle size for toughening a PS/PC polymer blend.