Carbon nanotubes (CNTs) were grown uniformity on the surface of carbon fibers to create hierarchical fibers by use of floating catalyst chemical vapor deposition. The tensile properties of CNTs grafted fibers were measured. The strength of grafted fibers decreased approximately 12% compared to raw carbon fibers in this process. A fiber pull-out test revealed that the interfacial shear strength (IFSS) in polypropylene composites improved by 35% by grafting CNTs onto carbon fibers. It indicates the use of hierarchical fibers in thermoplastic composites is effective due to the improvement of IFSS by mechanical interlocking between CNTs and the matrix. The CNT/fiber joint strength is the most critical property, and the experimental observations also revealed that the joint fracture was the major failure mode.