The effects of a compounding method and gamma-ray treated multiwall carbon nanotubes (MWCNTs) on the electrical conductivity of graphite/PET composites were investigated. We found that dispersion of MWCNTs in the PET phase plays a critical role in determining the electrical conductivity of graphite/PET/MWCNT composites. Dispersion and electrical conductivity were enhanced by a two-step method in which PET and MWCNTs are compounded in advance and the MWCNT/PET mixture is then compounded again with graphite. It was also observed that gamma-ray treated MWCNTs provide enhanced conductivity in the graphite/PET/MWCNT composite. The synergetic effect of the two-step mixing method and gamma-ray treatment made it possible to increase the conductivity of graphite/PET composites to a great extent with a very small amount of gamma-ray irradiated MWCNTs.