Batches of multi-walled carbon nanotubes having an average length of 2091 nm were aggressively tip-ultrasonicated to produce shortened carbon nanotubes having average lengths of 1689 nm, 1332 nm, 992 nm and 503 nm. Raman spectroscopic analysis confirmed that the shortened carbon nanotubes retained their crystallinity after the shortening process. Carbon nanotubes were then dispersed in the epoxy matrix using high-shear mixing technique (calendering). The mechanical properties were measured for the cured epoxy–0.1 wt% carbon nanotube nanocomposites having carbon nanotubes of different lengths. It was found that the nanocomposites containing long carbon nanotubes (2091 nm and 1689 nm) possess higher tensile strength, elastic modulus, fracture strain and fracture toughness as compared to nanocomposites containing short carbon nanotubes (1332 nm, 992 nm and 503 nm).