Numerical calculations of flow and injection pressures during injection molding of fiber-filled thermoplastics are compared to experimental measurements. The flow is modeled as a 2-D, nonisothermal, free-surface flow with a new viscosity model dependent upon temperature, pressure, and fiber concentration. The steady-state viscosity model is developed to account for the fiber-concentration and shear-thinning viscosities of the polymer based upon combining the Dinh-Armstrong fiber model with the Carreau viscosity model. The new model has four parameters, three from the Carreau model and one from Dinh-Armstrong for fiber concentrations. The new model calculates reasonably well the steady-state viscosity of fiber-filled polypropylene over the shear rate range of 0.01 s(-1) to 20 s(-1). The numerical work successfully describes the flow of fiber-filled polymers during injection molding using finite-difference solutions for the transport equations and marker particles to track the flow front. The comparisons between the calculated and measured pressure drops for an injection molded part were reasonable for the unfilled and fiber-filled polypropylene materials. The pressure drop comparison is very good for slow fill of a base case resin, Himont polypropylene, but not as good for fast fill of the resin. The pressure drop comparison is very good for fast fill of glass-filled resin, DSM polypropylene with 10% and 20% short fibers, but not as good for slow fill of the resin and resin plus fibers.