A dielectric sensor mounted in an extrusion die, was used to measure the dielectric properties of PVC melt during extrusion process. The dielectric permittivity and loss factor, of the polymer melt, were used to calculate the ionic viscosity and a dielectric viscosity index, which were correlated to the melt viscosity measured by slit rheometry equations. The viscosity of the PVC melt was varied by varying the level of chemical blowing agent (AZO) and processing aid (K400) in the polymer compound. The extrusion conditions; screw speed and heating zones, were fixed throughout the experiments. Strong correlation between the measured dielectric parameters and the melt viscosity was found to be consistent with established theoretical models. Therefore, this technique is capable of monitoring the viscosity of polymer melts accurately and reliably, in realtime, without disturbing the flow stream.