Carbon nanotubes (CNTs) with different orientation and lengths were grafted onto carbon fibers by catalytic chemical vapor deposition to produce hybrid preforms that were used to reinforce carbon/carbon (C/C) composites. Compressive property tests indicated that these C/C composites have obvious improvements in out-of-plane compressive strength, compared with pure C/C composites. Furthermore, the improvement has strongly influenced by the orientation and length of CNTs. Compared with entangled CNTs and radially-grown straight CNTs with smaller length, CNTs with a radial grafting morphology and longer length can reinforce the F/M interface and the whole matrix more efficiently and then increase the compressive strength better.