The shear viscosity of binary blends comprising polycarbonate (PC) and low-molecular-weight polystyrene (LPS) was examined. It was found that the viscosity of PC significantly decreases by the addition of L-PS. Although the dynamic mechanical properties indicated that L-PS is immiscible with PC, the morphology observation of the extruded strand clarified that shear-induced phase-mixing occurs. Furthermore, the viscosity decrease was found to be pronounced in the high shear stress condition, which is much more obvious than that by the addition of low-molecular-weight PC. Consequently, the addition of L-PS improves the flowability at injection-molding greatly.