A failure analysis of four carbon nanotube (CNT) wires comprised of 1-, 30-, 60-, and 100-yarns was conducted when subjected to constant tension and cyclic tension-tension loading conditions. Each wire had different controlling mechanisms of failure. Tensile and cyclic load-induced failures were related to the movement within yarns and/or among yarns in the CNT wires. The 1-yarn CNT wire exhibited a ductile fracture when constant tensile load was applied; recoverable deformation bands were observed on bending and straightening. The 30-yarn CNT wire showed a variant/independent fibrillar failure under constant tensile loading condition, while it failed by biaxial rotation, bend and twist under cyclic loading condition. The 60-yarn CNT wire resulted in a stake and socket fibre fracture when loaded to failure in constant tension; however, in the cyclic loading condition, the wire failed by kink band process. The 100-yarn wire failure mechanism was controlled by the surface wear in both constant tension and fatigue loading conditions. This failure analysis study presents detailed fracture surface features that can be used to diagnose the cause of failure, develop failure mechanisms, and improve the properties of CNT wires when used in real-life applications.