A novel carbon nanotube (CNT)-reinforced SiC matrix composite-fiber with excellent mechanical, electrical, and thermal resistant properties was fabricated by filling the macroscopic CNT fibers with a polycarbosilane precursor-derived SiC matrix at 1000 °C. The SiC matrix had a nanoporous and amorphous microstructure, uniformly distributed among the CNTs. The CNT experienced some compressive residual stresses due to the matrix shrinkage during processing, leading to densification of the fiber, increase in the hopping channel number, and strong CNT/SiC interfacial interaction. The composite fiber displayed a brittle-fracture response during tensile deformation, with the tensile modulus and strength ∼1.5 times higher those of pure fiber. The dominant fracture mechanism was a bridging effect of SiC on the CNTs that favored better load transfer during tensile deformation. Unlike that for the pure fiber, the mechanical performance of the composite fiber was well maintained after harsh treatment at 1000 °C in Ar, evidencing its excellent thermal resistant property. In addition, the electrical conductivity of the composite-fiber was ∼1 time higher than that of the pure fiber and 4–5 times higher than that of the PAN-based carbon fiber, mainly due to the denser fiber microstructure after SiC infiltration. Finally, the composite-fiber showed a better oxidation resistant property, demonstrating promising applicability under high temperature and oxidation conditions.