Ultrathin MnOOH nanofibers were synthesized on a large scale from diluted Mn(NO(3))(2) aqueous solution at room temperature. These MnOOH nanofibers were shape-reservedly converted into Mn(3)O(4) and MnO(2) nanofibers by post-heat treatment in air at 400 °C and 600 °C for 1 h, respectively. The morphology and crystalline structures of the nanofibers were characterized by electronic microscopes and x-ray diffraction. These nanofibers had good crystalline structures. These nanofibers were in bundles with a diameter of 25 nm composed of 3-5 nm fine crystalline nanofibers. The Mn(3)O(4) nanofibers had a specific surface area of 71 m(2) g(-1) and demonstrated highly catalytic degradation of the organic pollutant methylene blue with the assistance of H(2)O(2) at room temperature.