This study described the synthesis of hydrochloric acid (HCI)-doped poly (N-methylaniline) (PNMA) with carboxylic groups containing multi-walled carbon nanotubes (c-MWNTs) via in situ polymerization. Based on the pi-pi electron interaction between c-MWNTs and the N-methylaniline monomer and the hydrogen bond interaction between the carboxyl groups of c-MWNTs and imine groups of N-methylaniline monomers, N-methylaniline molecules were adsorbed on the surface of c-MWNTs and polymerized to form PNMA/c-MWNT composites. Scanning electron microscopy images showed that both the thinner fibrous phase and the larger block phase Could be observed. The individual fibrous phases had diameters from several tens to hundreds of nanometers, depending on the PNMA content. Transmission electron microscopy proved that PNMA/c-MWNTs composite fibrous phases were core (c-MWNT)-shell (PNMA) tubular structures. The structure of PNMA/c-MWNT composites was characterized by FTIR, UV-vis spectra, and X-ray diffraction patterns. The electrical conductivities of PNMA/c-MWNT composites were much higher than that of PNMA without c-MWNTs. (c) 2006 Wiley Periodicals, Inc.