Composites of polypropylene (PP) filled with multiwalled carbon nanotubes (CNTs) of various concentrations were prepared by a twin screw extruder using direct compounding (DC) method without and with ultrasonic treatment. In addition, a masterbatch of 20 wt% PP/CNT composites were prepared without and with ultrasonic treatment and diluted to the same concentrations as in the DC method without ultrasonic treatment. This is called the masterbatch dilution (MD) method. The rheological, electrical and mechanical properties were investigated. The microdispersion was determined using optical microscopy to correlate the processing, properties and structure. It was shown that the MD method provided better dispersion of CNT in PP matrix than the DC method. This was indicated by an increase of the storage modulus, viscosity, electrical and mechanical properties. The fractal dimension of CNTs, D, and the backbone fractal dimension, x, of the CNT network were determined by fitting the rheological data to the scaling model. The lower fractal dimension of CNT and higher backbone fractal dimension of CNT network in composites prepared by the MD method compared with composites obtained by the DC method indicated a better dispersion. Additionally, a lower D and a higher x values as well as the favorable effect on the morphology and mechanical properties were achieved when the ultrasonic treatment at an amplitude of 13 mu m was applied in the MD method, indicating an advantage in use of the ultrasonic treatment in preparing the PP/CNT masterbatch. (C) 2014 Elsevier Ltd. All rights reserved.