Although carbon nanotubes have been extensively studied since 1991, it is not clear the relations between the tubes' length and their composites' functional and mechanical performance. We in this study employed ball milling to reduce the length of multi-walled carbon nanotubes (MWNTs) from a few microns to hundreds of nm through milling for different time slots. The milled tubes demonstrated a 12.0% increase in surface area. When dispersed in an elastomer matrix, MWNTs showed two types of morphology: separately dispersed and clustered. The cluster size reduced with the ball-milling time. The ball milling obviously increased the composite fracture strain, while mild increase in tensile strength was observed. When MWNT/elastomer composites were compared with carbon black (CB) composites, both showed similar strength, but the former demonstrated markedly higher strength at low strain-a highly desired property for elastomers. The MWNT composites show a strong filler-filler interaction with an unobvious Payne effect. Although the ball milling slightly reduced the electrical and thermal conductivity of the MWCNT composites, they demonstrated much higher conductivity than the CB composites. The ball-milling reduced up to 18.2% of the composite internal heat rise. (C) 2013 Elsevier Ltd. All rights reserved.