The percolation threshold of carbon nanotubes (CNTs)/epoxy resin composites was simulated in the Bruggeman' Effective-Medium Theory based on experiment. Both distinct percolation effect and low percolation threshold in the aligned CNTs/epoxy composites were predicted. With the CNTs loading larger than the percolation threshold, the critical exponent of CNTs/epoxy composites rises rapidly with the increase of aspect ratio of CNTs. It is shown that the electrical conductivity of composites presents distinct aeolotropism, the percolation threshold is sensitive relative to the tiny change of the orientation factor, the aspect ratio, and the structure of CNTs in the composites matrix. The simulated results are consistent with the experimental results basically, and the discrepancy between simulated results and experimental results has been interpreted reasonably. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012