Highly conducting polyaniline (PANI)-multi-walled carbon nanotube (MWCNT) nanocomposites were prepared by in situ polymerization. The FTIR and XRD show systematic shifting of the characteristic bands and peaks of PANI, with the increase in MWCNT phase, suggesting significant interaction between the phases. The SEM and TEM pictures show thick and uniform coating of PANI over surface of individual MWCNT. Based on observed morphological features in SEM, the probable formation mechanism of these composites has been proposed. The electrical conductivity of PANI-MWCNT composite (19.7 S cm(-1)) was even better than MWCNT (19.1 S cm(-1)) or PANI (2.0 S cm(-1)). This can be ascribed to the synergistic effect of two complementing phases (i.e. PANI and MWCNT). The absorption dominated total shielding effectiveness (SE) of -27.5 to -39.2 dB of these composites indicates the usefulness of these materials for microwave shielding in the Ku-band (12.4-18.0 GHz). These PANI coated MWCNTs with large aspect ratio are also proposed as hybrid conductive fillers in various thermoplastic matrices, for making structurally strong microwave shields. (C) 2008 Elsevier B.V. All rights reserved.