We generalize Bruggeman effective medium theory to investigate the effective electrical and thermal conductivity of carbon nanotube composites. Both the nonlinear dependence of effective thermal conductivity on the nanotube volume fraction in nanofluids and very low percolation threshold for carbon nanotube/polyimide composites are well predicted. Numerical results show that although single-walled carbon nanotubes possess a higher thermal conductivity than multi-walled nanotubes, they induce less effective conductivity due to interfacial resistance. Moreover, the non-spherical shape of carbon nanotubes helps to achieve a large enhancement of the effective conductivity, and the use of disc-shaped particles gives a larger conductivity than the use of needle-shaped particles. (c) 2006 Elsevier B.V. All rights reserved.