The influences of free volume and temperature on the viscoelastic properties of polycarbonate (PC)/multi-walled carbon nanotube (MWNT) composites were investigated by positron annihilation lifetime spectroscopy (PALS) and dynamic mechanical analysis (DMA). Three methods, including PALS, DMA and differential scanning calorimetry were used to determine the glass transition temperature (T-g) of PC/MWNT composites. The experimental results indicated that the higher the MWNT contents, the lower the T-g, which attributes to the large free volume hole and the enhanced polymer mobility in PC/MWNT composites with higher MWNT contents. The effect of MWNTs on viscoelastic property has been investigated in detail. A direct linear relationship between fractional free volume and viscoelastic property has been obtained using the Williams-Landel-Ferry equation based on free volume theory, which indicates that the free volume plays an important role in determining the viscoelastic property. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.