In this present study, the in situ fabrication of a microfibrillar composite based on poly(ethlene terephthalate) (PET), polyethylene (PE), and carbon black (CB) is attempted. PET and CB were first melt mixed. The CB/PET compound and PE were subsequently melt-extrusted through a slit die and then hot stretched. The morphological observation of the as-stretched extrudate indicated that well-defined microfibers of CB/PET compound could be generated at appropriate CB contents and a fixed hot stretch ratio. In addition, CB was always selectivity located in PET. The microfibrillar CB/PET/PE composite has the potential to be a new electrically conductive polymer composite.