Manganese oxide nanofibers with layered structure and gamma-MnOOH structure have been prepared from a layered manganese oxide by using a self-assembling hydrothermal process. This process includes preparation of manganese oxide nanosheets by exfoliating the layered manganese oxide, self-assembling the nanosheets with a cationic surfactant into a nanocomposite, and transforming the nanocomposite into the nanofibers under hydrothermal conditions.