Porous anodic aluminum oxide templates with different channel structures have been synthesized and used to grow carbon nanotubes (CNTs) through the pyrolysis of ethylene at 700 degreesC. Aligned CNT arrays, Y-branched as well as novel dendriform. CNTs have been obtained and well characterized by scanning electron microscopy and high-resolution transmission electron microscopy. The amorphous structure, irregular and quasi-hemispherical ends as well as the branched junction have been revealed in detail. The disordered structure of the carbon nanotubes was assigned to the lattice mismatch between alumina and carbon species. A growth mechanism mainly based on the space-limited and self-catalytic functions of template channels has been proposed accordingly. (C) 2003 Elsevier Science B.V. All rights reserved.