Due to their excellent mechanical, electrical, and thermal properties, carbon fibers (CFs) have been used in nearly all-engineering fields, promoting vast research to improve their mechanical properties. However, the improvements of the mechanical properties of CFs are now saturated; thereby researchers pursue a new direction for improving the mechanical properties of the CF reinforced composites. On the other hand, carbon nanotubes (CNTs) have been regarded as a new generation reinforcement material, stimulating a considerable amount of research. However, the application of CNTs to polymer composites has brought many problems related with aggregation of CNTs that allow their low volume fraction in the composites. Thus, the hybridization of CNTs and CFs has been considered to be a versatile method to develop new advanced materials by hierarchically combining their excellent thermal, electrical, and mechanical properties at nano and micro scales . Direct growth of CNTs on CF surface is regarded as an effective hybridization method that can improve the reinforcing effect of CFs in composites and solve the dispersion problems of CNTs, becasue radially grown CNTs on CFs can improve the radial stiffness and axial tensile strength of CF-reinforced composites. In addition, the interfacial shear strength (IFSS) of polymer composites and the improved electrochemical performance as CF electrodes were also reported. In this presentation, we will report on a fabrication method of CNT-grafted CF without its degradation using bi-metallic catalysts and low-temperature grafting process . CF reinforced composites and carbon/carbon composites using CNT-grafted CF were manufactured and their mechanical properties such as modulus, tensile strength and flexural strength and electrical/thermal conductivity were characterized, demonstrating significant improvement. Finally, the mechanism behind such improvement will be presented in detail at the Conference.