Extremely high shear rate processing was applied to the compound system of acrylonitrilestyrene copolymer/polycarbonate (SAN/PC). The viscosity was measured against the shear rate The first non-Newtonian region, the second Newtonian region and second non-Newtonian region were observed in the compound system. The occurrence of these regions are very similar to those in the parent polymers, SAN and PC. For the calculation of the viscosity-shear rate curve, a concentric multilayer model was proposed. It gives good agreement between the calculated value and the measured one. A new mechanism for the occurrence of the non-Newtonian, second Newtonian, and second non-Newtonian regions was proposed. Nuclear spin-spin relaxation time measured on SAN, T2, seems to be consistent with the consideration that the occurrence of non-Newtonian region, second Newtonian region, and second non-Newtonian region are caused, respectively, by the disentanglement, near saturation of disentanglement associated with snapping of macromolecules, and reentanglement through recoiling of snapped macromolecules, and further snapping of the macromolecules, which is inconsistent with the proposed explanation.