Carbon nanotube (CNT) fibers are promising reinforcements in ceramic matrix composites where the service environments involve extremely high temperatures that are generally beyond 1000 °C. This work focuses on the thermal stability of a direct spun CNT fiber in vacuum and in a wide temperature (25-2000 °C). The microstructure, mechanical and electrical properties of the fibers as a function of the heat-treatment temperature were investigated. The results show that high temperature exposure could increase the defect density and loosen the packing state of the fiber, but enhance the graphitization degree of the CNTs. Accordingly, there was a ductile-to-brittle transition in the uniaxial tensile response as the heat-treatment temperature increased, and this was mainly a consequence of the failure mode transitions from localized shear to defect dominant fracture. The tensile modulus was enhanced, but the tensile strength was decreased after the heat-treatment. The former can be explained by the enhanced graphitization degree of CNT and the latter should be a result of the increased defect density. Finally, the electrical property of the fiber was degraded, due to the increased contact resistance of mutual CNTs.