Microstructure and damping characteristics were investigated for polycarbonate/multiwalled carbon nanotube (PC/MWCNT) composites. Dynamic mechanical analysis results show that the damping factor and the value of the energy loss fraction w are significantly increased. Especially, nearly 300% improvement in damping factor is observed in the temperature range from 140 to 150 degrees C. Positron annihilation lifetime measurements indicate that both the o-Ps lifetime and the free volume increase with increasing MWCNT content, leading to decrease in the glass transition temperature and increase in the damping properties. The relationships among the damping, mechanical property, and the free volume have been first observed, that is, the increase of the free volume brings about a reduction in tensile strength and an increase in damping. The interfacial friction slipping of MWCNTs and the free volume play an important role in determining the damping property of PC/MWCNT composites. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012