Multiwalled carbon nanotube (MWNT) nanocomposites with poly(ethylene terephthalate) (PET) were prepared via in situ polymerization. The refluxing time was more important factor than the sonication time for giving carboxylic groups onto the surface of MWNT. Acid-MWNT prepared was well dispersed in ethylene glycol, whereas the neat-MWNT agglomerated and sedimented at the bottom. The viscosity of the composites increased with the addition of MWNT, but PET/acid-MWNT composite showed lower viscosity than PET/neat-MWNT because of the damage of MWNT by acid treatment and copolymerization effect by the reaction between carboxylic groups of MWNT and PET. PET/acid-MWNT composite film showed lower surface resistivity than PET/neat-MWNT composite film. (c) 2005 Wiley Periodicals, Inc.