Poly(propylene) (PP)/chlorinated PP (CPP)/multiwalled carbon nanotube (MWNT) composites are prepared via melting blend with a MWNT masterbatch obtained by a solution process under ultrasonic irradiation. The effects of ultrasonic irradiation on the microstructure and the electric properties of the PP/CPP/MWNT composites are systematically investigated through a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), wide-angle X-ray diffraction (WAXD), and rheological measurements. Ultrasonic irradiation can remarkably decrease the volume resistivity of the PP/CPP/MWNT composites. The results of SEM and TEM show that the ultrasonic irradiation is beneficial to the dispersion of MWNTs in the PP/CPP/MWNT composites and the exfoliation of the MWNT agglomerates. When the MWNT content is more than 3.0%, ultrasonic irradiation can evidently increase the percent crystallinity of the PP/CPP/MWNT composites. The introduction of ultrasonic irradiation can increase the elastic modulus (G), viscous modulus (G), and complex viscosity (*) of the PP/CPP/MWNT composites at low frequency. Ultrasonic irradiation is also shown to improve the interfacial adhesion of MWNT and the interaction between the PP matrix and MWNT. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013