In the present study, it was attempted to fabricate a new conductive carbon black (CB) filled poly(ethylene terephthalate) (PET)/polyethylene (PE) in situ microfibrillar composite with a lower percolation threshold through selectively localizing CB particles in the surfaces of the PET microfibrils. The CB particles were first mixed with PE matrix, and then PET was added into CB/PE compound. Subsequently, the CB/PET/PE composite was subjected to a slit die extrusion, hot stretch and quenching process to generate in Situ PET microfibrils, in which CB particles moved to the surfaces of the PET microfibrils simultaneously. The morphological observation showed that the PET phases formed well-defined microfibrils, and CB particles did overwhelminaly localize in the surfaces of the PET microfibrils, which led to a very low percolation threshold, i.e., 3.8 vol%, and a good conductivity. The conductive network was built by the contact and overlapping of the CB particles coated PET microfibrils. In addition, the CB particles remaining in the PE matrix also contributed to the conductive paths, especially for the high CB loading filled microfibrillar composites. Because of the complexity of the distribution of CB particles, a high critical resistance exponent t (t = 6.4) exists in this conductive composite. To reveal the possibility of the migration of CB particles from PE to PET, the morphology of the CB/PET/PE composite mixed for different times was examined. It was found that, depending on the mixing, time, the CB particles gradually migrated from the PE matrix to the surfaces at first, and then to the center of the PET phases. The preferable distribution of CB particles was originated from several factors including interfacial tension, viscosity, molecule polarity, and mixing process. Furthermore, during the mixing process of the CB/PET/PE composite, the migration of CB particles to PET phase from PE matrix led to the increase of both the viscosity ratio of the dispersed phase to the matrix and the volume of the dispersed phases, thus resulting in larger dispersed CB/PET composite phase particles. (c) 2006 Elsevier Ltd. All rights reserved.