In this letter, the microstructural changes of catalytically grown carbon nanotubes are studied at high temperature and pressure by transmission electron microscopy (TEM). The nanotubes deform and one kind of ribbon-like nanostructure is transformed from carbon nanotubes directly when the annealing temperature is above 950 degrees C under 5.5 Gpa. It is suggested that pressure and temperature play an important role in the structural transformations of carbon nanotubes and ribbon-like nanostructure formation described here. (C) 1999 Acta Metallurgica Inc.