A new high-pressure slit rheometer has been designed in order to evaluate the pressure dependency on polymer melts viscosity. The new device can operate at pressures and temperatures up to 500 bar and 350°C. In the system, shear stress is measured by the pressure difference generated by polymer melt flowing through a rectangular slit die and shear rate is applied by controlling the sample's volumetric flow rate. The new design avoids correction calculations for exit and entrance flow effects. The system's pressure is independently controlled by using a slit-die pressurized at both the entrance and exit. Additionally, shear rates up to 2000 reciprocal seconds can be continuously imposed in a cyclic manner. The selected material to evaluate the new instrument's functionality is Polycarbonate. Viscosity curves were generated at various pressures, ranging from atmospheric to 500 bar, and the viscosity pressure coefficient from the Modified Cross-WLF viscous flow model was determined. Simulations using the measured viscosity, reflected that the required mold filling injection pressures are under estimated by a factor of three when the pressure dependence on the polymers melts viscosity is not included.