Multiwalled carbon nanotube/polycaprolactone nanocomposites (MWNT/PCL) were prepared by in situ polymerization, whereby as-received 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) on the MWNT. The micrographs of the fractured surfaces of the nanocomposites showed that the A-MWNTs in A-MWNT/PCL were better dispersed than P-MWNTs in PCL matrix (P-MWNT/PCL). Percolation thresholds of the P-MWNT/PCL and A-MWNT/PCL, which were studied by rheological properties, were found at similar to 2 wt % of the MWNT. The conductivity of the P-MWNT/PCL was between 10(-1) and 10(-2) S/cm by loading of 2 wt % of MWNT although that of the A-MWNT/PCL reached similar to 10(-2) S/cm by loading of 7 wt % of MWNT. The conductivity of the P-MWNT/PCL was higher than that of the A-MWNT/PCL at the entire range of the studied MWNT loading, which might be due to the destruction of pi-network of the MWNT by acid treatment, although the A-MWNT/PCL was better dispersed than the P-MWNT/PCL. The amount of the MWNT at which the conductivity of the nanocomposite started to increase was strongly correlated with the percolation threshold. (c) 2007 Wiley Periodicals, Inc.