The mechanical behavior of four carbon nanotube (CNT) wires, comprised of 1-yarn, 30-yarn, 60-yarn, and 100-yarn, was investigated under constant tension at two loading rates of 0.02 mm/sec and 2 mm/sec. Tests were conducted with both asfabricated and pre-stressed wires. The ultimate tenacity or apparent ultimate tensile strength of all four wires was found to be independent of loading rate, with those for the single-yarn wire about twice those of the multiple-yarn wires. Strain at a given stress level and failure strain of the multiple-yarn wires before pre-stressing were almost an order of magnitude larger than those for the single-yarn wire, and this difference was reduced considerably after pre-stressing. The failure mechanisms of 1-yarn wire or twisted individual yarns in multiple-strand wires involved ductile (necking) deformation and fibrillar breakage. Inner yarns in multiple-yarn wires initially failed at the same location, followed by outer yarns failing at different locations. Additionally, sliding occurred between individual yarns, and twisting of the yarns accompanied by surface wear took place in the multiple-yarn wires, which contributed to their failure at a lower load compared to the single-yarn wire.