We report the crystallization behavior of the amorphous carbon nanotubes at high temperature in this paper. Transmission electron microscopy and X-ray diffraction were performed to characterize the structures of the carbon nanotubes. The results reveal that the microstructure of the as-grown carbon nanotubes, prepared by the floating catalyst method, is roughly amorphous. The as-grown carbon nanotubes were annealed at high temperatures, and the crystallization behavior of the amorphous carbon nanotubes was investigated systematically. Because the carbon nanotubes have finite dimensions and tube-like shape, their crystallization behavior is completely different from the bulk amorphous carbons. The results reveal that the graphene layers in the annealed carbon nanotubes will form into a uniform two-dimensional turbostratic stack, a configuration of carbon nanotubes with the lower Gibbs free energy. A thermodynamic model is presented to explain the crystallization behavior of the amorphous carbon nanotube.