Blends of polycarbonate (PC) and highly branched polystyrene (HBPS) were prepared by melt blending. The steady rheological behavior of them was determined using a capillary rheometer, furthermore, the effect of shear rate, temperature and the blend component on the viscosity of the blends was discussed. The results showed that all the blends exhibit the nature of the pseudo-plastic fluids, and the viscosity of them decreases dramatically with the increase of temperature and does slightly with the increase of shear rate. At a fixed shear stress, the viscosity of the blends is decreased with the increase of the HBPS content. Microstructure studies using SEM showed that all the blends are characteristic of a two-phase morphology, with spherical droplets of the minor HBPS phase dispersed in the continuous PC phase.