A dual-sensor system is developed to measure the density and viscosity of polymer melts, simultaneously, during extrusion process. The sensory system utilizes a 2.25 MHz longitudinal ultrasound transducer in conjunction with a 2.25 MHz shear type ultrasound transducer placed in a modified extrusion die. The acoustic impedance of the polymer melt, corresponding to the longitudinal ultrasound waves (UTL), is used to measure the density of the polymer melt using established physical models. While, the acoustic impedance of the shear waves (UTS) is used to measure the viscosity of the polymer melt, utilizing the density measurements obtained by the longitudinal ultrasound transducer. The system was evaluated using polyvinyl chloride compounds containing various levels of foaming agent and processing aid at fixed processing conditions. The system measurements of the melt density was compared to the density of the extrudate, while the viscosity measurements were compared to the calculated values using slit-rheometry equations. The correlation coefficients between the ultrasound sensor and actual measurements were in the range of 92% and 96% for the melt density and viscosity; respectively.