Aggregation in polymer composites is one of the major obstacles in the carbon nanotubes (CNTs) applications. Authentic CNTs are known to have very good electrical conductivity and mechanical strengths. Surface functionalization can avoid aggregation and help dispersion of CNTs, but reduces CNT's electrical conductivities and mechanical strengths dramatically. It needs a good way to resolve the above dilemma situation; i.e., poor dispersion-good conductivity vs. good dispersion-poor conductivity. Herein, we demonstrate that in-situ polymerized polyaniline (PANI)-coated CNTs have good polymer matrix compatibility, and are superior electrically conductive fillers to nylon 6 composites. In this report, multi-walled CNTs (MWCNTs) were surface-modified with poly(acrylic acids) (PAA), followed by further coating with PANI. The electrical conductivity of (PANI-MWCNTs)-nylon 6 composite thin film was increased from 10-12 to 7.3 × 10-5 S/cm in the presence of 1 wt% PANI-coated MWCNTs prepared by physical mixing of PANI and PAA-grafted MWCNTs. When in-situ polymerized PANI-coated MWCNTs were added, the electrical conductivity of MWCNTs-nylon 6 composite was further enhanced by 3 orders to be 3.4 × 10-2 S/cm at the same 1 wt% loading of MWCNTs. Both Fourier-transformed infrared and uv-visible absorption spectra indicate that there exist very strong site-specific charge transfer interactions between the quinoid rings of PANI and MWCNTs, which results in the superior electrical conductivity of MWCNT-nylon 6 composite. © 2011 Elsevier Ltd. All rights reserved.