A template-synthesis technique based on self-ordered hexagonal nanopore alumina templates was applied for synthesizing highly ordered carbon nanotube arrays via thermal decomposition of ethylene with iron as catalyst electrodeposited into the bottom of the pores. It was found that the resultant carbon nanotubes in diameter and shape exactly reflect the pore of the template used. This shows that the diameter of nanotubes can be easily controlled and adjusted through the pore sizes of the templates, and the nanotubes with various shapes can be grown by applying templates with the same pore shapes as those of the tubes. This controllable synthesis method will be of great fundamental and practical interests.