Abstract: Carbon nanotubes (CNTs) were prepared directly on carbon fibers by chemical vapor deposition (CVD) method. The different process parameters such as catalyst, deposition temperature, hydrogen flow rate and distance between samples and intake were investigated in relation to morphology and structure. The samples were characterized and analyzed by SEM and high resolution transmission electron microscopy (HRTEM). Results show that CNTs in-situ grown on surface of CF is multi-wall structure. CNTs which grow on carbon fibers surface using Ni-catalyst are thin and uniform distribution. The diameter and length of CNTs using Fe-catalyst reduce and yield decrease when the deposition temperature rises from 600 ℃ to 750 ℃. The diameter and length of CNTs raise with hydrogen flow rate increasing. It is in favor of preparing high quality CNTs at 30 cm from fibers to intake, at which CNTs densely distribute on the carbon fiber surface and their diameter are uniform.