Viscosity measurement and the flow behavior prediction of reinforced polymer melts have been considered crucial for quality consistency in commercial products where highly filled polymer composites are used. In this study, a slit-die rheometer mounted on an injection molding machine was employed to measure the shear viscosity of a reinforced polymer melt by monitoring the pressure drop in the slit and the ram movement for various melt temperatures and shear rates. Mold filling experiment was also conducted using an injection molding process with two plaque-shaped parts of different thicknesses. Polycarbonate with 40 weight % of glass fibers was investigated in the experiment. A comparative numerical simulation was conducted as well using the measured viscosity in the current study. The pressure drops from the simulation and the experiments were compared, which reveals that the viscosity measured using a slit-die rheometer is relatively accurate.