The microstructural transformations of carbon nanotubes under high-pressure annealing have been studied by high resolution transmission electron microscopy. It is found that under 5.5GPa the microstructure of nanotubes has obviously changed at an annealing temperature of 770℃, while onion-like and ribbon-like structures formed at 950℃. The transformation from carbon nanotubes into quasi-spherical onion occurs in several steps, first the shells of nanotubes collapse, then spheres form and finally onion-like structures appear. The effects of high pressure and annealing temperature on microstructural changes are discussed.