In this study, the flow and rheological behaviors of low density polyethylene (LDPE) and single-phase LDPE/N-2 polymer/gas solutions at various gas contents were measured using a high-pressure slit-die rheometer. The resulting rheology data of LDPE and LDPE/N-2 were curve fit using the Cross-WLF model to obtain the seven material model constants, which were then used in the simulation of conventional and microcellular injection molding processes. The pressure effect on the shear viscosity of LDPE was also studied. The pressure dependency coefficient, D3, in the Cross-WLF model was determined by the use of simulation to best fit the experimental results. In addition, a three-dimensional plot of shear viscosity as a function of shear rate and pressure at various temperatures was constructed. Based on this three-dimensional viscosity-shear rate-temperature-pressure plot, the increase in viscosity due to the pressure effect was found to be more profound at high pressures, low temperatures, and low shear rates.