The interface layer thickness between two phases, i.e., the compatible domain between interfaces, is calculated in the high impact polystyrene (HIPS)alloys with poly(cis-butadiene) rubber (PcBR) by the intensity of small angle x-ray scattering. The phase structure and morphology of the HIPS alloys with PcBR are studied by scanning electron microscope images. The size of the phase structure is calculated by graph processing and its variation is discussed. The structure parameters of the small angle light scattering are analyzed. The authors found that the mechanical properties of the modulus and yield stress have a linear correlation with the composition and structure parameters of the alloys when PcBR is a dispersed phase.