Thermal and viscoelastic properties of composites of polypropylene (PP) filled with 0-8 vol % of vapor grown platelet carbon nanofibers (PCNF) were investigated. High shear mixing was used to disperse and distribute the nanofibers. Scanning electron microscope (SEM) was used to study the morphology of the composites that indicated the good dispersion of nanofibers within the PP matrix. Thermogravimetric analysis showed thermal stability enhancements due to the presence of PCNF in the PP matrix. DSC analysis indicated that the inclusion of nanofibers increased the melting temperature of PP matrix. By the incorporation of PCNF, the storage modulus increased whereas the mechanical loss factor (tan 8) decreased. The use and limitations of various theoretical equations to predict the storage modulus and tan 8 of the fiber reinforced composites have been discussed. Cole-Cole analysis has been carried out to understand the phase behavior of the nanocomposite samples. Thermal conductivity increases from 0.125 to 0.181 W/mK. Thermal conductivity values were compared with several theoretical and semi empirical models. The van Beek model showed a very good correlation to the measured values. (c) 2007 Wiley Periodicals, Inc.