On the basis of the previous observations that the ultrasonic signals are sensitive to the crystallization of polymers (Tatibouet and Piche, Polymer 1991, 32, 3147), we have expanded our efforts to study the detail relationship between the ultrasonic signals and crystallization process in this work. The nonisothermal and isothermal crystallization of virgin poly(ethylene terephthalate) (PET) and PET samples after degradation were studied by using a specially designed pressure-volume-temperature (PVT) device, with which an ultrasonic detector was combined. The results showed that the evolution of the ultrasonic signals not only can be used to probe the crystallization process but also can qualitatively characterize the crystallization rate, crystallinity, crystallite size, and amorphous. DSC measurement was used to verify such results. Ultrasonic signals could be as a complementary tool to polymer chain movement and well be applied to characterize the crystallization behavior. Furthermore, the ultrasonic measurement has the potential use to characterize crystallization of products in-line during processing (i.e., injection molding, micromoulding). (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 2731-2739, 2009