The effects of ultrasound with different frequencies on the degradation, crystallization behavior, and mechanical properties of polypropylene were investigated by means of intrinsic viscosity, gel permeation chromatography, dynamic rheological measurements, X-ray diffraction, differential scanning calorimetry, polarized optical microscopy, and tensile and impact measurements. The results demonstrated that the degradation extent of polypropylene (PP) increased with increasing sonication time, especially for the samples treated in more than 5min. Meanwhile, PP treated at 20 KHz was more easily degraded than other two frequencies. On the other hand, -form PP could be induced by ultrasonic irradiation, its amount increased as the sonication time increased to 3min and then decreased as the time further increased due to the degradation of PP. Likewise, -phase was more easily formed when PP was treated at 20 KHz. The results of mechanical properties showed that the toughness of PP could be enhanced by the formation of -phase, while the mechanical properties would be weakened by the substantial degradation under ultrasonic irradiation with higher intensity. Therefore, it was believed that the effects of both sonication time and frequency should account for the balance between toughening and degradation of PP. POLYM. ENG. SCI., 55:2566-2575, 2015. (c) 2015 Society of Plastics Engineers