The aim of this study is to investigate the effects of carbon nanotube (CNT) chemical properties, CNT content, and molding temperature on the morphology, electrical, and mechanical properties of the microfiber-reinforced polymer composites. These composites were prepared by extrusion and hot stretching the poly(ethylene terephthalate) (PET)/CNT phase in high density polyethylene (HDPE) matrix. Surfaces of the CNT were modified by purification with strong acid mixture (HNO3 : H2SO4 mixture 1 : 1 by volume) followed by treatment with poly(ethylene glycol) (PEG). In situ microfibrillar composites were prepared with untreated and modified CNT. Scanning Electron Microscopy (SEM) analyses indicated that CNT were preferentially located in PET phase of the composites. SEM micrographs of the hot-stretched composites pointed out the existence of PET/CNT microfiber structure in HDPE phase up to 1 wt % CNT loadings and the electrical resistivities of these composites were lower than 107 ohm/cm. Tensile strength values of the composites containing 0.75 wt % CNT increased from 44 to 52 MPa after PEG treatment due to the improved mechanical strength of PET/CNT phase. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013