This article is a portion of a comprehensive study on carbon nanofiber-reinforced thermoplastic composites. The thermal behavior and dynamic and tensile mechanical properties of polypropylene-carbon nanofibers composites are discussed. Carbon nanofibers are those produced by the vapor-grown carbon method and have an average diameter of 100 nm. These hollow-core nanofibers are an ideal precursor system to working with multiwall and single-wall nanotubes for composite development. Composites were prepared by conventional Banbury-type plastic-processing methods ideal for low-cost composite development. Nanofiber agglomerates were eliminated because of shear working conditions, resulting in isotropic compression-molded composites. Incorporation of carbon nanofibers raised the working temperature range of the thermoplastic by 100 degreesC. The nanofiber additions led to an increase in the rate of polymer crystallization with no change in the nucleation mechanism, as analyzed by the Avrami method. Although the tensile strength of the composite was unaltered with increasing nanofiber composition, the dynamic modulus increased by 350%. The thermal behavior of the composites was not significantly altered by the functionalization of the nanofibers since chemical alteration is associated with the defect structure of the chemical vapor deposition (CVD) layer on the nanofibers. Composite strength was limited by the enhanced crystallization of the polymer brought on by nanofiber interaction as additional nucleation sites.