We examined electrical properties of composites of carbon nanofibers (CNFs)/linear low-density polyethylene (LLDPE) in the range of mixing time. An addition of CNFs into LLDPE led to a decrease of electrical resistivity, and the large amounts of CNFs were required to reach the electrical percolation threshold for the longer mixing time. For the research of these phenomena, we examined the effects of mixing time on the size (length) and spatial distributions of CNFs in the composites. SEM micrographs revealed the size reduction of CNFs in a series of mixing times, although the spatial dispersion of CNFs became more uniform at longer mixing time. To describe the reduction of CNFs theoretically, we hypothesize the size distribution of CNFs obeys a governing population balance kinetics based on an irreversible dissolution process. For the process, we proposed a size dependent breakage rate coefficient proportional to the size of CNFs. The model prediction for the time evolution of size distribution of CNFs has been validated with the experimental measurement showing good agreement. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 2980-2987, 2009