Structural phase transformation from multiwalled carbon nanotubes to nanocr ystalline diamond by hydrogen plasma posttreatment was carried out. Ultrahigh equivalent diamond nucleation density above 1011 nuclei/cm2 was easily obtained . The diamond formation and growth mechanism was proposed to be the consequence of the formation of sp3 bonded amorphous carbon clusters. The hydrogen chemi so rption on curved graphite network and the energy deposited on the carbon nanotu bes by continuous impingement of activated molecular or atomic hydrogen are resp onsible for the formation of amorphous carbon matrix. The diamond nucleates and grows in the way similar to that of diamond CVD processes on amorphous carbon films. The present method of hydrogen treatment provides a controllable way for the CVD of high quality diamond films.