Multiwalled carbon nanotube/nylon-6 nanocomposites (MWNT/nylon) were prepared by in situ polymerization, whereby pristine MWNTs (P-MWNTs) and purified MWNTs (A-MWNTs) were used as reinforcing materials. The A-MWNTs were purified by nitric acid treatment, which introduced the carboxyl groups (COOH) to the MWNTs. The SEM micrographs from the fractured surfaces of the nanocomposites showed that the A-MWNTs were well dispersed in A-MWNT/nylon as compared to the P-MWNTs in P-MWNT/nylon. The percolation thresholds of P-MWNT/nylon and A-MWNT/nylon, which were studied by rheological properties, were found at - 1 wt % of the MWNTs. The conductivity of P-MWNT/nylon and A-MWNT/nylon were 3.15 X 10(-5) and 7.46 X 10(-5) S/cm by the loading of 5 and 7 wt % of MWNTs, respectively. Differential scanning calorimetry (DSC) showed that the crystallization temperatures of P-MWNT/nylon and A-MWNT/nylon (195 degrees C) increased more than that of pure nylon (189 degrees C) due to the nucleation effect of MWNTs. Wide-angle X-ray analysis indicated that the incorporation of MWNTs into the nylon matrix resulted only in a-crystalline structure, which were quite different from those reported for the clay/nylon nanocomposite (show both alpha- and gamma-crystals). It might be due to the different geometry between the MWNTs and the nanoclay platelets, which have one and two-dimensional geometry, respectively.