The ultrasonic degradation of polystyrene (PS) and ethyleneepropylene diene monomer (EPDM) melts was conducted in a specially designed reactor. A degradation model of polymer melts was proposed to explain the ultrasonic degradation, and degradation of polymer melts is treated as a mechanochemical process. Ultrasonic degradation kinetics obeys the equations: MTZMNCA e kt or ½h tZ½h NC ½h 0 ½h N e kt. The experimental results show that the molecular weight or intrinsic viscosity of polymer melts decreased with ultrasonic irradiation time and approached a limiting value, below which no further degradation took place. Dierent from ultrasonic degradation of polymer solutions, the scission of chains in polymer melts is random in the initial stage, and then non-random. FTIR spectra of samples ultrasonically treated and untreated confirmed that the initial product of ultrasonic degradation in polymer melts is a macroradical. The effects of ultrasonic intensity and reaction temperature on degradation of PS and EPDM melts were also investigated. The significant results showed that the degradation rate, the limiting value and degradation extent are correlated greatly with the experimental conditions.