Composites of polyethylene oxide (PEO) filled with carbon nanofiber (CNF) were prepared using solution mixing followed by melt mixing. The morphology and thermophysical properties of these composites and of pristine polyethylene oxide were analyzed by scanning electron microscopy, differential scanning calorimetry, thermogravimetric analysis, and dynamical mechanical analysis. Crystallization kinetic data were analyzed within the Avrami approximation for the primary stage of crystallization. The influence of CNF on the temperature dependence crystallization of PEO and the tendency of forming three-dimensional crystallites at higher crystallization temperature were observed. The nucleation features of CNFs dispersed within PEO were investigated by using Lauritzen-Hoffman nucleation theory. Crystallization activation energies were also computed; the results are in agreement with the Lauritzen-Hoffman theory. Thermogravimetric analysis and dynamical mechanical analysis showed a gradual increase of the thermal stability and of the storage modulus of the polymeric matrix due to the loading with CNFs and formation of a polymer-carbon nanofiber interface. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011