Carbon black (CB) filled electrically conductive in situ microfibrillar poly(ethylene terephthalate) (PET)/polyethylene (PE) composites (FCMC) with CB particles selectively localizing at the surfaces of PET microfibrils were successfully prepared through a slit die extrusion-hot stretch-quenching process. Resistivity-temperature behaviors of the FCMC samples were studied systematically during heating-cooling runs (HCR) with different top test temperatures. When the top test temperature was set as 140 degrees C, the resistivity abnormally increased during cooling below 100 degrees C, showing the cooling-induced resistivity increase. The room-temperature resistivity after one heating-cooling run was 4 orders of magnitude higher than that of the original samples. Thermal residual Stresses developed in the interfaces between PET microfibrils and PE matrix were responsible for the cooling-induced resistivity increase, which led to the damage of the conductive network. The top test temperature dominated the cooling-induced resistivity increase of FCMC. There was a critical temperature, 150 degrees C, above which the cooling-induced resistivity increase disappeared. A model was proposed to illustrate this cooling-induced resistivity increase. (C) 2008 Elsevier Ltd. All rights reserved.