The structure, energetics and thermal behavior of all the SiGe nanotubes in armchair and zigzag structures (n = 4-10) and two atomic arrangement types are investigated using the ab initio method and classical molecular dynamics simulations. Gearlike and puckering configurations of SiGe nanotubes are obtained. The simulation results indicate that large-diameter nanotubes are more stable than small-diameter ones. Moreover, the type 1 (alternating atom arrangement type) zigzag nanotubes are always more energetically favorable than the type 2 (layered atom arrangement type) zigzag nanotubes. During the melting process, the melting-like structural transformations from the initial nanotube to the compact nanowire take place first, and then the compact nanowires are changed into agglomerate structures at higher temperature. It is also found that the melting-like temperatures of Ge-substituted silicon nanotubes decrease with increase of the Ge concentration.