We demonstrate that nanosize diamond particles in a stable solid phase can act as nuclei for carbon nanotube (CNT) growth by chemical vapor deposition. This growth process is explained by a new growth mechanism, where the surface diffusion of carbon adatoms on the solid surface of nanoparticles plays an important role, contrary to the bulk diffusion in conventional metal catalysts. The use of diamond nanoparticles overcomes the issues of deactivation due to fusion with each other or reaction with substrate materials in the CNT growth using conventional metal catalysts and promotes high-density CNT growth from highly dense nanoparticles on any substrate.