The linear rheological properties of highdensity polyethylene (HDPE), polystyrene (PS), and HDPE/PS (80/20) blends were used to characterize their structural development during extrusion in the presence of ultrasonic oscillations. The master curves of the storage shear modulus (G) and loss shear modulus (G) at 200℃ for HDPE, PS, and HDPE/PS (80/20) blends were constructed with time-temperature superposition, and their zero shear viscosity was determined from Cole-Cole plots of the outof-phase viscous component of the dynamic complex viscosity versus the dynamic shear viscosity. The experimental results showed that ultrasonic oscillations during extrusion reduced G and G as well as the zero shear viscosity of HDPE and PS because of their mechanochemical degradation in the presence of ultrasonic oscillations; this was con-firmed by molecular weight measurements. Ultrasonic oscillations increased the slopes of log G versus log G for HDPE and PS in the low-frequency terminal zone because of the increase in their molecular weight distributions. The slopes of log G versus log G for HDPE/PS (80/20) blends and an emulsion model were used to characterize the ultrasonic enhancement of the compatibility of the blends. The results showed that ultrasonic oscillations could reduce the interfacial tension and enhance the compatibility of the blends, and this was consistent with our previous work.