In an attempt to understand the relationships between the evolution of ultrasonic signal and the change of molecular structure during processing, shear-induced chain orientation and its subsequent disorientation of high-density polyethylene (HDPE) melts with different melt indices were studied using a noninvasive and nondestructive ultrasonic technique. The molecular structural development during orientation and disorientation of these melts was manifested in the ultrasonic velocity. Two models were developed to describe the relaxation processes of orientation and disorientation, respectively. The effects of shear rate and temperature on the maximal degree of orientation and the relaxation time of orientation and disorientation were also explored. These results from the ultrasonic measurements were compared with ones obtained through rheological measurements, X-ray diffraction (XRD), and infrared dichroism measurements. The experimental results indicated that the ultrasonic technique was sensitive and promising for the real-time monitoring of the evolution of molecular structure during polymer processing. POLYM. ENG. SCI., 50:1140-1150, 2010. (C) 2009 Society of Plastics Engineers