Multibranched carbon nanotubes were grown by pyrolysis of acetylene in an anodic aluminum oxide (AAO) template with and without catalytic Co particles at the pore bottom. HRTEM structure analysis shows that both types of carbon nanotubes (CNTs) exhibit the same multiwall features, which consist of cylindrically stacked flakes, and the channel structures, either straight or multi branched, have no effect on the tube wall structure. The pore internal surface of the AAO template plays a catalytic role for the decomposition of acetylene. Due to the poor crystallinity of the present CNTs. their thermal stability in air is lower than that of CNTs by the carbon are method. but it is better than that of the refined mixed fullerenes. AFM examination reveals that the CNTs can be easily squashed by the interaction between the tip and sample: the height profile can be used to measure the tube wall thickness under contact mode. The formation of CNTs without catalyst deposition gives a reliable and convenient way to study the complex channel structure of the AAO template.