The objective of this study is to investigate the effect of ultrasonic reflection echo during polymer mixing process in the internal mixer. Phase change from solid to liquid, partially melted pellets, and distributing progress of CaCO3 powder during the melting process of low density polyethylene (LDPE) and the mixing process of the melted LDPE with a calcium carbonate (CaCO3) powder were successfully monitored by ultrasound. Melting and mixing completions were determined when ultrasonic reflection echo reached its upper stable level. The mixing period was proportional to the mass of added CaCO3 powder. The mean peak value of ultrasonic reflection L2f echo at the upper stable level decreased when the total amount of added CaCO3 powder increased. The presented ultrasonic technique can be utilized to optimize the melting and mixing processes, reduce cost and evaluate melting and mixing quality.