The transformation process of nanoribbons produced by the hydrothermal treatment in 10 M NaOH solution at 200 degrees C was investigated systematically via electron microscopy. Field emission scanning electron microscope (FE-SEM) observation showed that the treatment duration had a strong effect on the product morphology from the hollow nanotubes to nanoribbons. The details of transformation were studied by transmission electron miscroscopy. Some nanotubes assembled into clusters and grew into nanowires as the grain to form the nanoribbons, whilemost of nanotubes dissolved into the solution again.