Assessment of the potential of a polymer flood process for mobility control requires an accurate model on the viscosities of displacement fluids. Because most polymers used in enhanced oil recovery exhibit shear-thinning behavior, effective viscosity of polymer solution is a highly nonlinear function of shear rate. A reservoir simulator, including the model for the shear-rate dependence of viscosity, was used to investigate the shear-thinning effects of polymer solution on the performance of the reservoir in a five-spot pattern operating under polymer flood followed by waterflood. The model can be used as a quantitative tool to evaluate the comparative studies of different polymer flooding scenarios with respect to shear-rate dependence of fluids' viscosities. Results of cumulative oil recovery, injectivity, and distributions of polymer concentration and aqueous phase viscosity are presented for parameters of shear-rate dependencies, permeability, and injection rate. The results of this work have proven the importance of taking non-Newtonian behavior of polymer solution into account for the successful evaluation of polymer flood processes. Simulations carried in a 3D model suggest that the shear-thinning property improves injectivity considerably because the shear rate encountered near the injection well alters the viscosity of displacing fluid. However, it makes almost negligible differences in oil recovery because a significant increase in viscosity was developed at the fluid front in the reservoir. A thorough understanding of polymer rheology in the reservoir and accurate numerical modeling are of fundamental importance for the exact estimation on the performance of polymer flood.