Activated carbon (AC), a common carbon material, is employed as catalyst to synthesize carbon nanotubes (CNTs) through chemical vapor deposition (CVD) and detonation-assisted CVD methods. The results show AC can effectively catalyze CNT formation. From the microscopic observations on morphologies and structures of the formed intermediates, it is found that carbon-catalyzed CNT formation follows particle-wire-tube stepwise evolution mechanism, in which carbon nanoparticles first assemble into wire-like nanostructures, then evolve into nanotubes via particle-particle coalescence and structural crystallization. (C) 2010 Elsevier Ltd. All rights reserved.